FI124685B - A process for the production of sodium hydroxide from a waste stream for the production of pulp - Google Patents

Description

A process for the production of sodium hydroxide from a waste stream for the production of pulp

The present invention relates to a process for the production of sodium hydroxide from a waste stream of a pulp manufacturing process according to the preamble of claim 1.

According to such a method, a waste stream containing organic waste and sodium compounds is subjected to incineration at an elevated temperature to decompose the dissolved organic matter and recover the sodium.

10 The mill producing chemimechanical pulp, such as BCTMP pulp (or bleached CTMP pulp), usually processes its waste water in separate waste water treatment plants, such as activated sludge plants, and then incinerates the resulting sludge in its bark boilers. BCTMP plants are mainly independent dry bale pulp mills with their own biological wastewater treatment plants. These factories do not have 15 chemical recovery and recycling facilities.

There are alternative ways of treating waste water. Millar Westem's Meadow Lake plant in Canada was the first so-called. wastewater free BCTMP plant in the world. At this plant, the effluent to be removed from the process is led to its own sludge evaporator, where it is evaporated to high solids and the concentrate is incinerated in its own recovery boiler. The melt from the soda boiler is pelletized on cooling and stored in landfills. The melt contains the alkaline chemicals used in the process and the inorganic salts dissolved in the wood. So the plant is virtually a waste water-free plant, but there is no chemical recovery and no recycling. The BCTMP mills built by M-real in the 21st century, at Joutseno (2001) and No 25 at Kaskinen (2005), have been integrated into sulphate pulp mills for concentrate processing and chemical recovery. BCTMP mills evaporate their effluent co into high dry matter (> 45%) in their co-evaporators and from here concentrate x is pumped into the black liquor of the pulp mill located in the same mill site where it is

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passes through a pulp mill evaporator to a boiler for incineration. In a boiler <8 30, the organic matter burns to form carbon dioxide and water (-> CO 2 + H 2 O) and the inorganic materials melt at the bottom of the boiler. This melt is dissolved in water to form a green liquor (Na 2 CO 3 + Na 2 S / major products). The sodium contained in the alkaline chemicals used by the BCTMP plant is thus recovered by leaching (green liquor) in the recovery boiler. The cooking chemicals for the sulphate pulp mill are sodium hydroxide 2 (NaOH) and sodium sulfide (Na2S). Soda boiler combustion has reducing conditions whereby sulfur is recovered directly in the sulfide form. White liquor contains sodium hydroxide (NaOH) and sodium sulfide (Na2S) as the main components.

5 The pulp mill uses most of the green liquor to produce white liquor, but some of the green liquor is oxidized (Na2CO3 + Na2S → Na2CO3 + Na2SO4) and fed to the BCTMP mill. Oxidation eliminates sodium sulfide, which would cause problems at the BCTMP plant because it consumes hydrogen peroxide, which is used to bleach pulp. Sodium sulfate (Na2SO4) is a neutral salt that passes '' as a dead load '' through the CTMP-10 mill process and ends up in the concentrate back into the recovery boiler where it is reduced back to sodium sulfide.

Sodium carbonate is not an effective chemical in chemical pulp cooking for lignin removal (delignification). Thus, green liquor is not suitable for use in the absorption or cooking of chemical pulping. Green liquor is used to make white liquor through lime seasoning. In this process, burnt lime (CaO) is added to the green liquor, and as a result of causticization reactions, the carbonate precipitates as calcium carbonate, with sodium forming sodium hydroxide. The lime is regenerated in a lime kiln by burning (CaCC> 3 -> · CaO).

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In the manufacture of a chemimechanical pulp, such as CTMP, the removal of lignin by chips absorption does not take place, the alkaline treatment (impregnation) merely softens the lignin and is treated with carbohydrates (hemicellulose) for the subsequent refining step (chipping the chips). The pH of the impregnation solution is lower (pH 9-12) than in the preparation of chemical pulp o 25 (pH 14). As a result, sodium carbonate can also be used to impregnate chips in the manufacture of a chemimechanical pulp.

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x Integrated BCTMP processes utilize oxidized green liquor and lye (NaOH), typically purchased externally, for pulp impregnation.

S 30 Peroxide bleaching uses alkaline alkali. Coniferous sulphite is commonly used for softwood impregnation.

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In addition to the integrated solution, the recovery of sodium chemicals can also be carried out independently at the CTMP plant. One application, namely the so-called. According to the Alrec process (Alkali 3 recovery process), the waste liquor concentrated to a dry matter content of about 65% is incinerated. as droplet burning under oxidizing conditions (excess oxygen) at 1000-1200 ° C.

In Alrec combustion, unlike in recovery boilers, the conditions throughout the combustion chamber 5 are oxidizing (over oxygen). The oxygen content of the Alrec flue gas is 4-6% by volume. In batch boiler combustion, the stepwise importation of air regulates the combustion conditions in the various parts of the boiler so that, for example, reducing conditions exist in the furnace.

- COD (= dissolved organic matter) is destroyed (-> CO2 + H 2 O), and - sodium in the spent liquor is recovered as sodium carbonate (Na 2 CC> 3).

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Sodium carbonate can be recycled to absorb chips and bleach CTMP pulp, where it can also be used as a source of alkali along with lye.

In the high-temperature process described, the sodium bound to the organic material of the effluent concentrate is evaporated to a gas phase where it is present in part in Na 2 O form and partly in elemental sodium (sodium boiling point 883 ° C). The Alrec process delay at a firing temperature of 1000-1200 ° C is only a few seconds, after which time the flue gas in the combustion chamber is rapidly cooled to 600 ° C. In this case, the sodium compounds in the gas state directly subside as solid powdered sodium carbonate. This prevents the formation of adherent molten sodium carbonate and thus the fouling of the walls.

The solid sodium carbonate (ash) obtained from the process is dissolved in water, the impurities are removed by filtration, and the NaCl solution is recycled to absorb chips and bleach pulp.

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The above process is described in more detail in WO 2005/0687111 (Rinheat Oy).

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S 30 There are limitations to the state of the art. In our experiments, we have found that the replacement of lye by sodium carbonate in absorption decreases the pH of the solution and the absorption and softening of hardwood chips is reduced, which tends to increase the specific energy consumption in the milling process. The usability of the recovered alkali, a 100% sodium carbonate solution, to be absorbed is limited by the increase in specific energy in 4 refining. According to our test runs, the effect of carbonate on the specific energy of refining would at least depend on the quality objectives (paper technical properties) and possibly the wood species (birch / aspen) of the CTMP pulp to be produced.

5 Sodium sulfite (NaiSOs) sulfite used for absorption in the manufacture of softwood CTMP is oxidized to neutral sulfate (NaiSCL) in the above-described oxidizing Alrec firing. Sodium sulfate is a neutral salt and is not suitable as an alkaline chemical for absorption or bleaching. This means that the Alrec process is not suitable for the recovery and recycling of sodium sulfite without separate solutions for sulfite 10 recovery.

There are also restrictions on the use of sodium carbonate as an alkali in peroxide bleaching. Without the presence of a lye, the pH remains low in terms of bleaching efficiency.

It is an object of the present invention to provide a novel solution for the recovery of alkali in chemimechanical and mechanical pulping. Another object of the invention is, for example, to extend the usability of the alkaline chemical recovered by the Alrec process.

The invention is based on the idea that sodium hydroxide is produced from a waste stream of a fibrous pulp manufacturing process containing organic waste and bound sodium and sodium compounds by adding borate or a similar boron-containing substance to a process stream such as a waste stream or impregnation solution. Addition of a boron compound gives sodium hydroxide produced from sodium and sodium compounds, borate? 25 by auto-causticization reaction and subsequent hydrolysis.

o Carousing is a reaction described in the literature as early as the 1970s (see Jan ^ Janson, The Use of Unconventional Alkali in Cooking and Bleaching - Part 1. A New Approach to Liquid Generation and Alkalinity, Paper and Wood 59 (6- 7), pp. 425-430 (1977), -τ—— 30 The Use of Unconventional Alkali in Cooking and Bleaching - Part 2. Alkali Cooking of

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lo wood with the use of borate. Paper and Wood 59 (9), pp. 546-557 (1977) and U.S. Patent No. 4,116,759.) According to Janson's observations, conventional separate causticization with burnt lime could even be completely avoided in sulfate soup by addition of borate to 5 soup broth which autocauses to produce sodium hydride. in the melting of the recovery boiler melt, ie in the preparation of green liquor.

However, factory tests conducted by Enso Gutzeit in 1982 did not confirm 5 initial expectations about the profitability of the process.

About 20 years later, Honghi Tran re-examined borate auto-acidification reactions and showed that the lye yield from borate was twice that of Jansson. According to the reaction equations presented by Tran, one mole of tetraborate (NaiEECE) 10 produces 8 moles of lye (NaOH) instead of the 4 moles reported by Jansson. (Tran, H; Mao, X; Cameron, J; Bair, C.M., Pulp Pap. Can. 1999, 100 (8), 35-40). This had a significant impact on the profitability of borate carousing as it reduced the required borate dose by half. Tran's research has led to practical applications of partial borate auto-causticization in some sulphate cellulose mills.

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In his article Borate autocausticizing: a cost effective technology (Pulp & Paper Canada 103: 11 (2002), pp. 16-22, JMA Hoddenbagh et al.) Describe two factory experiments carried out 15 years after the Enso experiments, in which the auto-causticization reaction was used for both chemical recovery and Based on the results, the alkali produced by the borate auto-causticization process can potentially be used in CTMP pulp bleaching to replace externally purchased sodium hydroxide.

Combining borates with a pulping process, e.g., to provide boron-containing alkaline cooking solutions, has recently been described in the patent literature, of which International Patent Publication WO 2004/025020, wherein N for the preparation of chemical eucalyptus pulp, integrates with a borate-carbonate is used. The solution is based, at least in part, on auto-causticization, whereby part of the x-auto-causticized cooking chemicals are used for cooking and / or oxygen delignification, or temporal bleaching steps such as peroxide bleaching without conventional causticization of S 30. Anthraquinone is used as the delignification catalyst. U.S. Patent Publication No. 2005/0155730, for its part, describes a high yield process,

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wherein the chemical softwood pulp is prepared using a quinone catalyst in a chips impregnation step having at least a partial pH below 7, or in a low-sulfide cooking medium containing mainly borate, sodium hydroxide and 6-carbonate. The alkaline broth is prepared without separate causticization with calcium oxide or calcium compounds.

International Application Publication Nos. WO99 / 63152 and WO99 / 63151 disclose the use of borate to improve the efficiency of a 5 aqueous calcination reaction.

It has been surprisingly found in the present invention that autocausing reactions also occur under oxidative conditions and at high temperature combustion, such as an Alrec-type combustion process.

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According to the invention, the solution is carried out in that the autocausing reaction is carried out by burning the waste stream in oxidizing conditions at such a high temperature that at least part of the sodium is evaporated (b.p. 883 ° C). In particular, it is carried out at a temperature at which sodium evaporates and is mainly in the oxide form.

By reacting the sodium and its compounds released from the combustion of the organic matter with the borate mainly in the gas phase, the sodium hydroxide is produced in the ash leaching step. The reaction effluent can be used as an impregnation solution, e.g. in the BCTMP process or other chemimechanical defibration and as an alkali source in peroxide bleaching.

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The process can be used to recover at least partially sodium hydroxide, which is suitable for alkaline sodium compounds suitable for the pulp manufacturing process.

More particularly, the process according to the invention is essentially characterized by what is set forth in the characterizing part of claim 1.

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cp cd The use of the invention is characterized by what is claimed in claim 18.

The invention provides considerable advantages. Thus, the invention is generally applicable for use in combustion processes, which typically have a relatively high

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the liquor concentrated to a concentration of the substance is incinerated under oxidizing conditions (i.e. excess oxygen) at at least about 950 ° C. In particular, the at least partial conversion of sodium carbonate to sodium hydroxide according to the invention improves the usability of the alkali recovered, for example, by the Alrec process, which in turn reduces the chemical cost and energy requirement for subsequent pulping of the chips. The availability of alkali in peroxide bleaching is improved.

The invention enables a waste-free, independent (non-integrated) BCTMP and mechanical pulp mill with recovery and cost-effective recycling of alkaline chemicals. The thermal energy from the incineration of organic waste is used as steam in the process.

The present invention will now be explored in greater detail by means of a detailed description with reference to the accompanying drawing, in which the solution according to the first embodiment of the invention is shown in a flow diagram.

As stated above, the present invention combines auto-causticization with a high temperature incineration process whereby sodium bound to organic matter in the wastewater is reacted at least partially directly with borate under oxidative conditions in the gaseous phase to oxidize the Na 2 > 3) the mixture. It should be noted that the invention is not limited to this particular description.

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In one embodiment of the invention, the metaborate or metaborate-forming compounds are incorporated into the waste stream and the liberated sodium and sodium compounds are reacted with the borate at a temperature so high that the sodium evaporates and is mainly in the oxide form. Borate compounds can be "incorporated" into the waste stream by directly adding δ 25 to that stream or by adding them, for example, to chemical or mechanical defibration

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the impregnation solution with which they pass through the process and are included in the defibration co (effluent).

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According to one embodiment, the solids content of the effluent to be introduced for auto-causticization is S 30 at least 45% by weight, preferably at least 55% by weight, most preferably at least 60% by weight.

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According to another embodiment, the effluent concentrate is dried to a powder which is fed as a dry substance for incineration.

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Preferably, the effluent, concentrated to a high dry matter of e.g. at least about 60% by weight, especially at least about 63% by weight or at least 65% by weight, is incinerated in the presence of oxygen and borate or a compound forming it at least 950%. ° C.

The waste solids typically contains both an organic and an inorganic moiety. The weight ratio of these can vary within wide limits, usually about 3: 1 to 1: 1.

According to a preferred embodiment, the auto-causticization is carried out at a temperature of at least 1000 ° C, preferably 1000 to 1250 ° C.

"Oxidizing conditions" herein means that there is excess oxygen throughout the combustion process so that there are no reducing conditions anywhere in the combustion chamber.

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Preferably, the borate is added to the wastewater or impregnation solution prior to incineration to such an extent that the Na: B molar ratio of the effluent stream is at least about 3: 1. Preferably, the Na: B molar ratio is from about 3: 1 to about 50: 1, most preferably from about 5: 1 to about 35: 1.

The inorganic borate is added at least predominantly in the form of sodium metaborate or sodium tetraborate or hydrates thereof. Other boron compounds are also possible.

According to the invention, the waste water stream to be treated is typically obtained as waste water from pulp pulping in alkaline conditions. In particular, the effluent stream comprises the removal of impregnation, i.e. the impregnation of the raw material for chemimechanical or mechanical pulping, the waste stream of alkaline peroxide bleaching of the pulp, or a combination thereof. The waste stream may thus be derived, for example, from the production of groundwood, pressure groundwood, pulp or o co chemical pulp.

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The incineration waste stream mainly contains organic compounds already dissolved in the pulp production process, and the chemically bound sodium, which is thus derived from the impregnation of the raw material or from the use of alkaline peroxide bleaching or both.

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oxidized green liquor or oxidized white liquor derived from sodium-containing chemicals such as sodium carbonate, sodium hydroxide and / or chemical pulping.

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According to one embodiment, the incineration of the borate-containing (organic) waste stream is carried out in two stages,) the actual combustion is carried out in the first stage under oxidizing conditions above 1000 ° C, followed by rapid cooling of the flue gases below 600 ° C to directly solidify the sodium compounds. phase (Na2CO3 and borate compounds such as Na3BC3).

The ash from the incineration of the waste stream is recovered and dissolved in water to produce sodium hydroxide. At the same time, the metaborate (NaBO2) is regenerated.

Typically, as provided above, the calculated sodium hydroxide content of the ash is - depending on the organic matter and the sodium content of the waste water - about 1-75%, preferably about 5-70%, especially about 10-50%, by weight of the dry matter.

The auto-causticization is carried out under hypoxic conditions with oxidizing conditions throughout the combustion chamber. Typically, the oxygen content of the flue gas discharged is 4-6% by volume.

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Oxygen is introduced into the incineration stage as an oxygen-containing gas stream, such as air or oxygen enriched air.

The incineration can be carried out, for example, as a droplet burner, whereby the o 25 concentrate of the effluent to be used for auto-causticization is broken down into droplets. The average droplet size may be e.g.

ιΊ. about 0.1 to 5 mm, preferably <1 mm, and particularly preferably <0.1 mm.

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x The process described above can be used generally in the pulp manufacturing process

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used to recover, at least in part, the alkaline sodium compounds used as S 30 sodium hydroxide.

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In the preparation of chemimechanical pulps, such as BCTMP, partially causticized ash is dissolved in water, insoluble inorganic oxides (dregs) are removed by filtration or centrifugation and the resulting alkaline solution can be used directly, without further treatment by conventional lime causticization, impregnation and peroxide bleaching.

The invention can be implemented e.g. The apparatus 5 has a combustion chamber 1 and a cooling chamber 2, which, as shown, are sequentially aligned so that the combustion gases from the combustion chamber can be conducted to a cooling chamber where they can be cooled by a cooling gas such as a cooling line or recycled flue gas. The incinerator is provided with an inlet nozzle 3 for the waste water concentrate to be incinerated, through which the concentrate can be fed into, for example, small droplets as a vapor-dispersed mist.

The cooling gas is conveyed to the cooling chamber 2 from the inlet 4. At its bottom, the cooling chamber 2 is provided with a carbonate outlet 5 and a cooling gas and cooled fuel outlet 6.

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The process effluent containing the metaborate or the boron-forming compound is preferably evaporated to at least 45% dry matter in the process evaporator. It is then passed through the feed nozzle 3 to the combustion chamber 1, where it is incinerated, for example, at a temperature above 1000 ° C.

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The combustion process is described in more detail in WO 2005/068711 (Rinheat Oy).

The delay in the combustion chamber 1, at high temperature, is only a few seconds and the flue gas from the combustion chamber 25 is rapidly cooled in the cooling chamber 2 to below 600 L, whereupon the sodium compounds directly subside from the gas to a solid and the cd can be removed ash.

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Upon dissolution of the ash in water, the formed sodium orthoborate (trisodium borate,) <8 with water reacts with water to form sodium hydroxide, while the metaborate is regenerated as shown in Schemes 1 and 2:

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(1) Na3CO3 + NaBO2 - * · Na3BO3 + CO2 Metaborate Orthoborate 11 (2) Na3BO3 + H2O -> 2 NaOH + NaBO2

Borates are completely water-soluble compounds, and the regenerated metaborate is transported in the absorption solution to the impregnation and thence to the removal of the impregnation with waste water through evaporation back to the oxidizing incineration. The borate content in the effluent to the evaporation plant is kept at a constant level by the addition of make-up.

As stated previously, the combustion is carried out in the combustion chamber in excess oxygen (i.e., excess oxygen 10), whereby the combustion conditions in the chamber are considered to be completely oxidizable.

The leaching of the ash can also be carried out in the cooling chamber 2. In this case, the cooling chamber is fed with water from one of the four. The feed water can be either pure water or an aqueous solution such as a solution obtained from recycling. With the help of water, a liquid film can be formed on the surface 15 of the chamber, in which the alkali metal carbonate contained in the chilled fuel gases is dissolved.

Example 20 Under laboratory conditions, an evaporator concentrate with 52% organic matter and 48% inorganic dry matter was burnt overnight. The combustion temperature was 1100 ° C. Sodium metaborate was mixed with the concentrate before combustion.

Table

Ash Ash Water Ash Water Titrated δ% Na2CO3 pH of solution NaOH concentration___ mg / g ash ___ mg / g ash o Concentrate 43.1 719 11.8 40

Concentrate + NaBO 2 48.4 109 12.9 385 £ Na / B mol / mol = 3: 1 δ 25 ° LO The Na 2 CO 3 content of the ash was determined with a TOC carbon analyzer. The algal concentration was determined by titration of the ash solution based on SCAN-N 30:85.

Claims (18)

A process for producing sodium hydroxide from a waste water stream derived from a process for the production of fiber pulp, which stream contains organic waste and sodium bound thereto, according to which process 5 - waste the stream of water is concentrated to a high dry matter content, is incinerated under oxidizing conditions to decompose the organic waste and the sodium compounds, and - the ash obtained from the waste stream combustion stream is taken up and dissolved in water to produce sodium hydroxide, whereby borated or a compound which forms borate is fed to the waste stream. prior to combustion and the sodium compounds are led to an autocoustic reaction together with borate or a compound which forms borate, characterized in that the borate-containing organic waste stream is led to a combustion treatment carried out in two steps, Initial combustion is carried out in an initial stage under oxidizing conditions at a temperature above 1000 ° C, after which the combustion gases formed during combustion are rapidly cooled to a temperature below 600 ° C to sublimate the sodium compounds from gas phase directly to solid phase. 20 Process according to claim 1, characterized in that borate is incorporated into the waste stream by directly supplying the waste stream. 't 5 3. A process according to claim 1, characterized in that borate is incorporated into the waste stream by introducing the chemical solution used in the cp fiber fiber production. X cc 25 4. A process according to any one of the preceding claims, characterized in that a very borate is applied to the Na: B molar ratio of the waste water stream to at least 3: 1, preferably to 3: 1 to 50: 1 and most preferably to 5: 1 to 35: 1st o c \ j Process according to any one of claims 1-4, characterized in that the causticization reaction takes place at a temperature of at least 950 ° C, at which temperature the sodium containing the waste stream is gas phase and mainly in the form of sodium oxide. 5 6. A process according to any one of the preceding claims, characterized by borate borate, the waste water stream or the chemical solution of the defibration is supplied at least mainly as sodium metaborate or sodium tetraborate or as hydrates thereof. 7. A process according to any of the preceding claims, characterized in that the waste stream comprises a waste water stream from fiber pulp production which is carried out under alkaline conditions. 8. A process according to claim 7, characterized in that the waste water stream comprises a waste stream of reject from raw material impregnation in chemical or mechanical pulp manufacture, or a waste stream from an alkaline peroxide bleaching of fiber pulp. 15 9. A process according to claim 8, characterized in that the waste stream is derived from wood pulp, pressure grinding pulp, refining pulp or chemical refining pulp. 10. A process according to claim 8 or 9, characterized in that the waste stream contains mainly organic compounds dissolved in connection with mass production and sodium, which is bound to these compounds in connection with raw material impregnation or alkaline peroxide bleaching or bed, which sodium is derived from sodium , such as sodium carbonate, in cp sodium hydroxide or oxidized green liquor or oxidized white liquor. co Er 11. A process according to any of the preceding claims, characterized in that CL autocoustication is carried out at a temperature of at least 1000 ° C, preferably 1000-m [1250 ° C] and under oxidizing conditions. O) o o c \ j Method according to any of the preceding claims, characterized in that the delay in the combustion chamber is up to about 0.1 - 10 seconds, in particular to about 0.5 - 5 seconds. 13. A process according to any of the preceding claims, characterized in that the autocousticization is carried out in a combustion chamber, wherein oxygen is in excess during the entire combustion process so that reducing conditions do not occur anywhere in the combustion chamber. Process according to claim 13, characterized in that oxygen is supplied in the combustion stage as an oxygen-containing gas flow, such as air or oxygen-enriched air. Process according to any of the preceding claims, characterized in that the dry matter content of the wastewater brought to the autocausticizing process is at least 45% by weight, preferably at least 55% by weight and most preferably at least 65% by weight. Process according to any of the preceding claims, characterized in that the borate-containing wastewater concentrate brought to the autocustomization is dispersed to form droplets. Process according to any of the preceding claims, characterized in that the borate-containing waste concentrate brought to the combustion is in the form of a dry powder. The use of the process according to any one of claims 1 to 17 to utilize the alkaline sodium compounds used in the fiber pulp preparation at least partially in the form of sodium hydroxide. co X X Q. LO 00 LO O O o C \ J