SE536944C2 - Method for producing sodium hydroxide from an effluent from fiber pulp production - Google Patents

Description

The 536,944 BCTMP factories built in the 2000s by M-real at Jutseno (2001) and Kaskinen (2005) are, with regard to concentrate treatment and recycling of chemicals, integrated with sulphate pulp mills.

The BCTMP plants evaporate their waste water in a cascade of evaporators to a high proportion of dry matter (2450/0), and from the evaporator section the concentrate is pumped to a mixing zone for mixing with black liquor received from a pulp mill located in the same plant area. After passing through the evaporation unit of the pulp mill, the mixture thus obtained is burned in a soda recovery unit. In the soda recovery unit, the combustion of the material generates carbon dioxide and water (-> CO 2 + H 2 O) and the inorganic ingredients form a melt at the bottom of the recovery unit. This melt is dissolved in water, which produces green liquor (Na2CO3 + NaZS / main products). Consequently, the sodium, which forms part of the alkaline chemicals used in the BCTMP factories, is recycled as part of the process of dissolving the melt (green liquor) in the soda recovery unit.

The cooking chemicals used in a sulphate pulp mill are sodium hydroxide (NaOH) and sodium sulphide (N aZS). The environment during combustion in the soda recovery unit is reducing, which is why sulfur is recycled directly in sulphide form. The main components of white liquor are sodium hydroxide (N aOH) and sodium sulfide (NaZS).

A pulp mill uses a larger part of the green liquor to produce white liquor, but part of the green liquor is oxidized (Na2CO3 + Na2S -> Na2CO3 + Na2SO4) and led to the BCTMP plant. Oxidation is a way of removing sodium sulfide, which would otherwise cause problems at the BCTMP plant because it consumes hydrogen peroxide, which is used in bleaching the pulp. Sodium sulfate (NazSOáj) is a neutral salt that passes as a "dead weight" through the process at the CTMP plant and ends up in concentrated form back in the soda recovery unit, where it is reduced back to sodium sulfide.

Sodium carbonate is not an effective chemical for equation removal (ie for delignification) in the boiling of chemical masses. Consequently, green liquor is unsuitable for use in impregnation or boiling in the production of chemical pulp. White liquor is produced from green liquor by causticizing with lime. In this process, calcined 536,944 lime (CaO) is added to the green liquor, and as a result of causticizing reactions, the carbonate precipitates as calcium carbonate, while sodium forms sodium hydroxide. The lime is regenerated by burning (CaCOß -> CaO) in a lime sludge re-incinerator.

In the production of chemical mechanical pulp, such as CTMP, lignin is not removed in the step of impregnating wood al by using alkaline treatment (impregnation), instead the lignin is softened and carbohydrates (hemicellulose) are treated before the next refining step (defibration of wood chips). The pH of the impregnation solution or impregnation liquor is lower (pH 9-12) than in the production of chemical pulp (pH 14). Consequently, it is possible to also use sodium carbonate for the impregnation of wood ice in the production of nuclear mechanical pulp.

In integrated BCTMP processes, oxidized green lye from the pulp mill and lye (NaOH), which are typically purchased from external suppliers, are used in the impregnation of hardwood. The alkali used in the peroxide bleaching is lye. Sodium sulphite is primarily used for impregnation of softwood.

In addition to using an integrated solution, it is also possible to perform a separate recycling of sodium chemicals at CTMP factories. According to one embodiment, namely the one known as the Alrec process ("alkali recovery process"), the waste liquor which is concentrated to a high proportion of dry matter of about 65% is incinerated using drop burning under oxidizing conditions (oxygen supply). ) and at a temperature of 1000-1200 ° C.

In Alrec combustion, in contrast to combustion a soda recovery unit, the conditions throughout the combustion chamber are oxidizing (excess oxygen). In Alrec combustion, the proportion of oxygen in the combustion gas is 4-6 vol ° / o. During combustion in a soda recovery unit, the combustion conditions are regulated by introducing air, in phases, into different parts of the unit in such a way that the conditions in the stack, for example, are reducing. Therefore - COD (dissolved organic matter) is destroyed 536 944 (-> CO2 + H 2 O), and - sodium from the waste sludge is recovered as sodium carbonate (N azCOß).

It is possible to recycle the sodium carbonate to be used in impregnating the wood chips and in bleaching the CTMP pulp, where it can also be used, in addition to lye, as a source of alkali.

In the high temperature process described above, the sodium in the waste water concentrate, which is bound to the organic material, is evaporated to its gas phase, where it is present partly in the form of Na 2 O and partly as elemental sodium (boiling point of sodium is 883 ° C). The delay of the residence time in the Alrec process is only a few seconds at a combustion temperature of 100 DEG-100 DEG C., after which period the combustion gases from the combustion chamber are rapidly cooled to 600 ° C. In this case, the gaseous sodium compounds are sublimed directly as solid powdered sodium carbonate. This prevents the creation of sticky molten sodium carbonate and thus possible contamination of the walls.

The solid sodium carbonate (ash) formed in the process is dissolved in water, impurities are removed by filtration and the NazCO fi solution is recycled to the impregnation of wood chips and the bleaching of pulp.

The process described above is reported in more detail in the published international patent application number WO 2005/068711 (Rinheat Oy).

There are limitations to the prior art. In our experiments we have discovered that when lye (sodium hydroxide) is replaced during the impregnation step with sodium carbonate, the pH of the impregnation solution is lowered and impregnation and softening of hardwood chips decreases, which tends to increase the specific energy consumption during refining. If the specific energy generated during refining increases, it limits the usefulness of the recovered alkali in the impregnation step, i.e. the 100% sodium carbonate solution. Our test runs suggest that the effect of carbonate on the specific energy of refining depends at least on the quality of the CTMP resin to be produced (technical grades for paper) which is 536,944 desirable and possibly on the proportion of different types of wood (birch / aspen).

The sulfite in sodium sulfite (N a 2 SO 3), which is used in softwood CTMP production, oxidizes during the above-mentioned oxidizing Alrec combustion process to a neutral sulfate compound (Na 2 SO 4). Sodium sulfate is a neutral salt and is not suitable as an alkaline chemical for impregnation or bleaching. Thus, the Alrec process is not suitable for the recovery and recycling of sodium sulfite, but separate procedures for the regeneration of sulfite.

There are also limitations associated with the use of sodium carbonate as the alkali in peroxide bleaching. Without the presence of lye, the pH values remain low and the bleaching effect is reduced.

The object of the present invention is to provide a completely new solution for the recovery of alkali in chemical mechanical and mechanical pulp production. Another object of the present invention is, for example, to extend the usefulness of alkaline chemicals recovered using the Alrec process.

The present invention is based on the concept of producing sodium hydroxide from a waste stream or waste of a fibrous pulp production process. The stream used typically includes organic waste and sodium and sodium compounds bound to the waste. Thus, borate or a corresponding boron-bearing material (hereinafter referred to as a "boron compound") is added to a waste stream or impregnation solution.

The addition of the boron compound makes it possible to generate sodium hydroxide, which is formed by sodium and sodium compounds, by a borate autacousticization reaction and subsequent hydrolysis.

Autacoustication is a reaction described in the literature as early as the 1970s (Sejan Janson, “The Use of Unconventional Alkali in Cooking and Bleaching - Part 1. A New Approach to Liquid Generation and Alkaliriity”, Paperíja Pm: 59 (6-7) , pp. 425- 430 (1977), “The Use of Unconventional Alkali in Cooking and Bleaching - Part 2.

Alkali cooking of Wood With the use of borate ”. Paperíja PM 59 (9), pp. 546-557 (1977) and U.S. Pat. 4,116,759. 536 944 According to Janson's observations, a separate caustication using steep lime can also be completely avoided during sulphate boiling by adding borate to the cooking liquor, which by the causticizing reaction produces sodium hydroxide in connection with the dissolution of the melt, i.e. the production of green liquor. However, factory tests performed by Enso Gutzeit in 1982 did not confirm initial expectations about the process's profitability.

About 20 years later, Honghi Tran re-examined autobaustic reactions with borate and showed that the exchange of lye from borate was double that described by Janson. According to the reaction formulas presented by Tran, one mole of tetraborate (Na 2 B 4 O 7) gives 8 moles of lye (NaOH), instead of 4 moles as Janson assumed. (Tran, H .; Mao, X .; Cameron,] .; Bair, CM., Paáü ana 'Paper Canada 1999, 100 (8), 35-40).

This discovery had a substantial effect on the profitability of autobausticating with borate, as it showed that only half of the original dose of borate was needed. Trans studies have resulted in partial co-caustic with borate being used in certain sulphate pulp mills.

In an article entitled “Borate autocausticizing: a cost effective technology” (Paåb d? Paper Canada 103: 11 (2002), pp. 16-22,] MA Hoddenbagh et al. Describe two factory tests that were performed 15 years after the Enso test and in which an auto-caustic reaction is used in both the recovery and bleaching of chemicals.Based on the results, alkali produced using the auto-caustic process with borate may possibly be used in bleaching CTMP pulp and substituted sodium hydroxide purchased from external suppliers.

Incorporation of borates into a pulping process, for example for the purpose of generating boron-alkaline cooking liquors, has recently been described in the patent literature, as exemplified by the published international patent application no. WO 2004/025020.

That document shows a process in which borate carbonate boiling is integrated with the recovery of chemicals in the production of eucalyptus chemical pulp.

This solution is at least partly based on autocausification, during which some of the cooking chemicals, which are regenerated using autocaustion, are used for boiling and / or oxygen delignification or during alkaline bleaching steps, such as peroxide bleaching without conventional causticizing. Anthraquinone is used as the 536,944 catalyst for the delignification.

Published U.S. Pat. 2005/01 55730 describes a high yield process in which chemical hardwood pulp is produced using quinone catalyst in the step of wood ice impregnation, in which the pH value is at least temporarily below 7, or in a cooking liquor having a low sulphide level and which mainly comprises borate, sodium hydroxide and sodium carbonate. An alkaline cooking liquor is prepared without separate causticization using calcium oxide or calcium compounds.

Two published international patent applications WO 99/631 52 and WO 99/63151 describe how the efficiency of a calcination reaction is improved by the use of borate.

In the present invention, we have surprisingly discovered that autacoustic reactions occur even during combustion performed under conditions of oxidation and high temperatures, such as under the conditions prevailing during an Alrec-type combustion process.

The solution is implemented, according to the new technology presented, in such a way that the autacoustic reaction is carried out by burning a waste in a combustion chamber in an oxidizing environment and at a temperature high enough to evaporate at least part of the sodium (boiling point of sodium is 883% ).

In particular, the operation is carried out at a temperature at which sodium evaporates and is mainly in oxide form (N azO). By making the sodium and its compounds, which during combustion are released from organic material, subject to a reaction with borate mainly in the gas phase, it is possible to produce sodium hydroxide when the ash typically containing sodium orthoborate, during a dissolution step, is dissolved or suspended. in water. The outcome of this reaction can be used as an impregnation solution, for example in a BCTMP process or other core mechanical defibration, and as a source of alkali in peroxide bleaching.

It is possible to use the present method for the recovery of alkaline 536,944 sodium compounds, which are at least partly in the form of sodium hydroxide, and which compounds are suitable for the fibrous pulp production process.

More particularly, the method according to the present invention is characterized by what is stated in the characterizing part of claim 1.

The use according to the present invention is characterized by what is stated in claim 20.

Significant advantages are achieved with the present invention. Thus, the invention is generally suitable for use in incineration processes in which, typically, waste liquor which is concentrated to a relatively high proportion of dry matter is incinerated under oxidizing conditions (i.e. in excess oxygen). The temperature is preferably at least about 950 ° C. in particular, at least a portion of the sodium carbonate is converted to sodium hydroxide which improves the usefulness of alkali recovered, for example, during the Alrec process. This, in turn, reduces the cost of chemicals and reduces the energy consumption required for defibrating wood chips, which follows impregnation. The usefulness of this alkali in peroxide bleaching is improved.

The present invention makes it possible to operate an independent (non-integrated) BCTMP plant and a mechanical pulp mill, which is free of waste water, and has means for recycling alkaline chemicals and cost-effective recycling.

Thermal energy generated during the incineration of organic waste is used as steam in the process.

Brief Description of the Drawing In the following, the present invention is examined in more detail by means of a detailed description and the accompanying drawing. The process flow diagram shown in the drawing represents an embodiment of the present technology. 536 944 The figure of the drawing shows a schematic block diagram of an embodiment of an equipment by means of which embodiments of the present invention can be implemented.

Detailed Description of the Invention In the present context, the terms "effluent" and "waste water" will be used broadly synonymously to refer to a stream / stream containing organic residues and sodium compounds which come from a fibrous pulp production process.

As described above, in the present technology, autocaustion and burning at a high temperature are combined to create a process in which sodium is bound, typically chemically bound, to the organic material of an effluent (waste water), is subjected to, in the gas phase and during oxidizing conditions, a reaction with a boron compound. It seems that under such conditions, sodium is caused to at least partially react directly with borate, or another boron compound, probably instead of with carbon dioxide. As a result, a mixture of lye (NaOH) and sodium carbonate (Na 2 CO 2) is formed during the dissolution of the ash, which is recovered in the process. It should be noted that the present invention is not limited to this exemplary model described herein.

According to one embodiment of the present invention, monaborate or compounds which form metaborate are incorporated into the effluent, and the sodium and sodium compounds released during processing are reacted with the borate at a temperature high enough to evaporate sodium which is then substantially in oxide form (N 2 O 2). There may also be some elemental sodium. It is possible to incorporate the borate compounds into the effluent by adding them directly to the effluent or by adding them, for example, in an impregnation stream of chememechanical or mechanical defibration, with which they are carried through the process and form part of the effluent from the defibration (i.e. waste water fl fate). ).

Preferably, the effluent is first concentrated to a high proportion of dry matter. According to a 536,944 embodiment, the proportion of dry matter of an effluent which is subject to autocaustion is at least 45% by weight, preferably at least 55% by weight, preferably at least 60% by weight, in particular at least 63% by weight, or even at least 65% by weight. O.

According to another embodiment, the concentrated waste water is dried to a powder, which is fed into the combustion as a dry substance.

Preferably, a residue concentrated in a high proportion of dry matter of, for example, at least about 60% by weight / o, especially at least 63% by weight or at least 65% by weight, is burned in the presence of oxygen and borate or a compound which forms borate ( boron compound ”), at a temperature of at least 950 ° C.

Typically, the dry matter of the waste comprises both an organic and an inorganic part.

The weight ratio between it can vary within wide ranges, generally it is about 3: 1. . .1: 1, but these are not absolute limits.

According to a preferred embodiment, autocaustion is performed at a temperature of at least 1000 ° C, preferably 1000-1250 ° C.

Here, “oxidizing conditions” or “oxidizing environment” means that during the entire or substantially entire combustion process, there is an excess of oxygen in order to prevent reducing conditions from occurring in any part of the combustion chamber. Oxygen is in excess of oxidizable compounds in the processed stream.

Most suitably, the amount of borate or other boron compound added to the waste water or impregnation solution before incineration is large enough to ensure that the Na: B molar ratio in the waste stream is at least 3: 1. Preferably, the Na: B molar ratio is about 3: 1 ... 50: 1, preferably about 5: 1 ... 35: 1.

When using borate, at least a major portion of the inorganic borate is added as sodium metaborate or sodium tetraborate or as hydrates thereof. Other boron associations are also possible.

According to the present invention, the effluent to be treated is typically generated 536,944 as waste water from fibrous pulp production which takes place under alkaline conditions.

In particular, the waste water stream comprises the effluent from an impregnation step, i.e. impregnation, of the raw material for chemical mechanical or mechanical pulping, or the waste stream from alkaline peroxide bleaching of fi berm pulp, or a combination thereof. Thus, the waste stream can come from the production of, for example, abrasive pulp, pressurized abrasive pulp (PGW), refiner pulp or chemical mechanical refiner pulp.

The waste water stream which is the subject of incineration mainly comprises organic compounds which are dissolved during the mass production, and sodium which is chemically bound to these compounds. Accordingly, the sodium comes from sodium-bearing chemicals used in the impregnation of raw materials or alkaline peroxide bleaching or both, such as sodium carbonate, sodium hydroxide and / or oxidized green liquor or oxidized white liquor coming from chemical pulping.

According to one embodiment, the combustion treatment is carried out, of an (organic) waste stream comprising borate or to which borate is fed, in two steps, the actual combustion taking place in the first stage under oxidizing conditions and at a temperature of more than 1000 ° C, after which the combustion gases generated during the combustion, which contains sodium in oxide and elemental form, is rapidly cooled to a temperature below 600 ° C, in order to sublimate the sodium compounds from their gas phase directly to the solid phase (sodium carbonate and especially borate compounds, such as Na3BO3).

The ash, which is generated from the combustion of the waste stream, is recovered and dissolved in water with the intention of producing sodium hydroxide. At the same time, the metaborate is regenerated (N aBO 2).

Typically, according to the measures described above, the ash contains a proportion (calculated) of sodium hydroxide which - depending on the organic material and the proportion of sodium in the waste water - is about 1-750 / 0, preferably 5-700 / 0, especially about 10-500 / 0, by weight dry matter.

Auto-caustic is performed under conditions of excess oxygen, whereby oxidizing conditions apply throughout the combustion chamber. Typically, the proportion of oxygen in the departing combustion gas is 4-6% by volume.

Oxygen is led to the combustion stage as a gas stream comprising oxygen, such as air or oxygen-enriched air.

It is possible to carry out the incineration as droplet incineration, whereby concentrated waste water which is led to autocaustion is dispersed to form droplets. The average droplet size may be, for example, about 0.1-5 mm, preferably <1 mm and more preferably S0.1 mm.

It is possible to use the methods described above generally to recover alkaline sodium compounds which are used in the fibrous pulp production process, at least in part as sodium hydroxide.

In the preparation of chemical mechanical pulp, such as BCTMP, ash, partially causticized in the preparation, is dissolved in water, the insoluble inorganic oxides (precipitate) being removed by filtration or centrifugation, and the alkaline solution generated can be used directly, without further treatment with traditional lime caustic, as a source of alkali for impregnation and peroxide bleaching.

The present invention can be implemented, for example, in an equipment arranged as shown in the drawing. The equipment comprises a combustion chamber 1 and a cooling chamber 2, which according to the drawing are arranged one below the other in such a way that it is possible to direct the combustion gases, which are generated in the combustion chamber, to the cooling chamber, in which it is possible to cool them by means of cooling gases. , such as cooling air or circulating combustion gas, which is led into the cooling chamber.

The combustion chamber is equipped with a nozzle 3 for feeding the concentrated waste water to be burned, through which nozzle it is possible to feed the concentrate, for example as a mist which comprises droplets dispersed by the use of steam.

The cooling gas can be led into the cooling chamber 2 through an inlet nozzle 4.

The bottom of the cooling chamber is equipped with an outlet nozzle 5 for removing 12,536,944 carbonate, and with an outlet nozzle 6 for removing cooling gases and cooled combustion gases.

Outflows derived from a process comprising metaborate or boron compounds forming metaborate are evaporated in an evaporation unit as an internal process, preferably with the intention of producing a dry matter content of at least 45%.

It is then passed through feed nozzle 3 into the combustion chamber 1, where it is burned, for example, at a temperature above 1000 ° C.

The combustion process is described in more detail in the published international patent application no. WO 2005/068711 (Rinheat Oy).

The residence time in the combustion chamber 1, at a high temperature, is only a few seconds and the combustion gas coming from the combustion chamber cools rapidly in the cooling chamber 2 to below 600 ° C, whereby the sodium compounds sublime directly to solid material and can be removed as ash.

When the ash is dissolved in water, the sodium orthoborate (trisodium borate) reacted with water reacts with the wild sodium hydroxide and, at the same time, the metaborate is regenerated according to formulas (I) and Na, co, + Naßo, Naßo, + co, metaborate orthoborate (2) Na3BO3 -l- HZO -> ZNaOH + NaBOZ Borates are completely water-soluble compounds and regenerated metaborate is moved in the impregnation solution to the impregnation step and from there on, in the effluent from the impregnation together with waste water, through the evaporation evaporator unit. The proportion of boron in the waste water that goes to the evaporation unit is kept at a constant level by additive stacking.

As previously pointed out, the combustion in the combustion chamber is carried out with excess oxygen, in order to ensure that the combustion conditions in all parts of the chamber are oxidizing. It is also possible to carry out the dissolution of the ash in the cooling chamber 2. In that case water is fed into the cooling chamber via the inlet nozzle 4. The water thus fed can be either pure water or an aqueous solution, such as a solution generated from the recirculation . With the aid of water, an aqueous film can be formed on the surface of the chamber, in which film it is possible to dissolve the alkali metal carbonates contained in the cooled combustion gases.

Example A concentrate from an evaporation unit, which concentrate has a proportion of organic material of 52% and a proportion of inorganic material of 48% of the dry matter, was burned with excess air under laboratory conditions.

The combustion temperature was 1100 ° C. Prior to firing, sodium metaborate was mixed into the concentrate.

Table Part NazCO CO pH value of Titrated proportion Ash exchange 0 of ash aqueous NaOH / 0 mg / g ash solution of ash mg / g ash Concentrate 43.1 719 1 1.8 40 Concentrate + NaBOZ; Na / B 48.4 109 12.9 385 mol / mol I 3: 1 The proportion of Na2CO3 of ash was determined using a TOC carbon analyzer.

The proportion of lye in the aqueous solution of ash was determined by titration, which was based on the standard SCAN-N 30:85. 14

Claims (18)

10 15 20 25 536 944 PATENT REQUIREMENTS A method of producing sodium hydroxide from an effluent from a fibrous pulp production process, which effluent comprises organic waste and sodium bound thereto, according to which method - the effluent is concentrated, - the concentrated effluent is burned under oxidizing conditions to decompose it. the organic waste and sodium compounds and to produce an incineration residue, - the residue is dissolved in water to produce sodium hydroxide, - borate or a compound forming borate is incorporated into the effluent before said combustion, and - the sodium compounds together with said borate or said compound forming borate are made into subjected to an autocaustic reaction carried out at a temperature so high that sodium evaporates, characterized in that - the borate-bearing organic effluent is subjected to a combustion treatment in two stages, the actual combustion taking place in the first stage under oxidizing conditions and at a temperature of more than 1000 ° C, after which the combustion gases generated during the combustion are rapidly cooled to a temperature of less than 600 ° C to sublimate the sodium compounds from their gaseous phase directly to their solid phase, and that the combustion residue comprises ash generated during the combustion of the effluent, which is recovered and dissolved or suspended in water to produce aqueous sodium hydroxide. Method according to claim 1, characterized in that borate is incorporated into the effluent by adding it directly to a waste water atten desolate. 15 10 15 20 25 536 944 3. A method according to claim 1, characterized in that borate is incorporated into the effluent by adding it to a chemical solution used in the pulp production, Method according to one of the preceding claims, characterized in that sufficient borate is added to make the molar ratio Na 2 S in the effluent at least 3: 1, preferably 3: 1 ... 50: 1 and most preferably 5: 1 ... : 1. Method according to any one of claims 1-4, characterized in that the causticizing reaction takes place at a temperature of at least 950 ° C, at which temperature sodium contained in the effluent is present in evaporated form, in a gaseous phase of the reaction, wherein said sodium is mainly in the form of sodium oxide, Method according to any one of the preceding claims, characterized in that the borate is added to the waste water fl or the chemical solution of defibration, at least mainly as sodium metaborate or sodium tetraborate or as hydrate thereof. Method according to one of the preceding claims, characterized in that the waste comprises a waste water waste from the fibrous pulp production which is carried out under alkaline conditions. Method according to claim 7, characterized in that the effluent comprises an effluent stream from impregnation of raw materials in chemimechanical or mechanical pulping, or the effluent comprises a waste water fl fate of an alkaline peroxide bleaching of fibrous pulp. Method according to Claim 8, characterized in that the output comes from the production of mechanical wood pulp or abrasive pulp, pressurized abrasive pulp, refiner pulp or chemical mechanical pulp pulp. Method according to claim 8 or 9, characterized in that the effluent contains mainly organic compounds which are dissolved in connection with mass production, and sodium, which is bound to these compounds in connection with the impregnation of raw materials or alkaline peroxide bleaching or both parts, and which sodium comes from the sodium-bearing chemicals used, such as sodium carbonate, sodium hydroxide or oxidized green liquor or oxidized white liquor. 16 10 15 20 536 944 Method according to one of the preceding claims, characterized in that the autacousticization is carried out at a temperature of at least 10 ° C, preferably 10 12-1250 ° C and under oxidizing conditions. Method according to one of the preceding claims, characterized in that the borate-bearing organic effluent is subjected to combustion in an combustion chamber and the residence time thereof in the combustion chamber is about 0.1-110 s, in particular about 0.5-5 s. . Method according to any one of the preceding claims, characterized in that - the autacousticization is performed in a combustion chamber, and ~ there is an excess of oxygen to prevent reducing conditions from occurring in any part of the combustion chamber. 14. A method according to claim 13, characterized in that the oxygen is fed into the combustion stage as a gas flow comprising oxygen, such as air or oxygen-enriched air. Method according to one of the preceding claims, characterized in that the proportion of dry matter of the waste water which is the subject of auto-causticization is at least 45% by weight, preferably at least 55% by weight, more preferably at least 60% by weight, and in particular at least 650 / 0. Method according to one of the preceding claims, characterized in that the boron-bearing waste water concentrate, which is the subject of auto-caustic, is dispersed to form droplets. 17. A method according to any one of the preceding claims, characterized in that the borate-carrying effluent concentrate, which is led to combustion, is in the form of a dry powder. Use of a method according to any one of claims 1-17 for the recovery of alkaline sodium compounds, at least in part in the form of sodium hydroxide, which are used in the pulp production process. 17