The present invention relates to a method for evaporating bleaching effluents in association with the partial or complete combustion of cellulose spent liquors in a reactor.

Background and problems

From an environmental point of view, bleaching pulp without using chlorine-containing chemicals (TCF) permits process solutions which are of great interest. Thus, the discharges of pollutants in bleaching effluents can be greatly reduced, especially by returning the bleaching filtrates in countercurrent to the recovery stage. Bleaching without chlorine- containing chemicals is preferably carried out using various combinations of oxygen, hydrogen peroxide, peracetic acid and ozone. Developments within this field are currently taking place very rapidly, and pulp brightnesses close to, and above, 90% ISO have been reported for softwood kraft pulps . Swedish Patent SE 469 387, for example, describes various processes for carrying out TCF bleaching. It is well known that bleaching with peroxides is sensitive to the content of transition metals and alkaline earth metals in the pulp and the bleaching liquors. For example, an acidic ozone stage can wash out magnesium which it is important to retain for selective peroxide bleaching. In addition, an ozone stage can mobilise transition metals, which metals can catalyse peroxide decomposition and carbohydrate breakdown in a subsequent peroxide bleaching stage. It is common practice to add complexing agents of the EDTA or DTPA type in association with the peroxide bleaching of chemical pulp.

The content of metals in the pulp can be decreased by means of an acidic wash, with or without complexing agents. Addition of the alkaline earth metals which are important for the bleaching result can

be effected directly downstream of the acidic washing stage.

A commonly occurring alternative is to adjust the pH with an acid and add complexing agents in a washing stage prior to the peroxide stages. With the use of EDTA and a pH of 6, most of the transition metals are chelated while the magnesium content is not affected.

In the modern bleaching plant, which is tightly closed, an effluent arises which cannot be recirculated in its entirety within the bleaching plant due to enrichment of non process elements and dissolved organic compounds. Thus, even in the "closed" bleaching plant, part of the stream has to be bled off and worked up. By virtue of various process solutions in the bleaching plant, described in more detail, for example, in SE 469 387, the stream which has to be bled off can be minimized and it should be possible to achieve flows of less than 5 m ³ /ADMT. This process stream, termed Q filtrate below, contains compounds which are harmful for the pulp production process and has to be dealt with separately. Several different alternatives have been discussed and published, of which the following can be mentioned: 1. Recirculation of the filtrate to the recovery boiler dissolver for dissolution and conversion of the smelt to green liquor. Normally, weak liquor and/or condensate is/are used for this task. However, the quantity of Q filtrate which can be added to the liquor stocks in this way is limited in most cases. 2. Evaporation of the whole or parts of the filtrate volume by means of a separate evaporation stage or in an existing black-liquor evaporation stage. This solution demands substantial ^* extension to the plant or, alternatively, investment in new evaporation apparatus. The concentrate is incinerated together with the black liquor and the contaminants are expelled by means of green-liquor

clarification/filtration. The condensate may need to be freed from COD/BOD before being reused. 3. Precipitating metals by means of adjusting the pH and filtering. The pH adjustment can be carried out using alkali which is normally present in the pulp mill such as sodium hydroxide, green liquor, white liquor and/or calcium compounds. The disadvantages of this method are the need to invest in a special filter and the difficulty in achieving a good filtration result. The filtration of filtrates containing complexing agents may require new developments to take place in the filtration field. It is also doubtful whether the filtered liquid achieves the desired degree of purity.

There is thus a need for simple systems for dealing with bleaching filtrates from tightly closed bleaching plants. The present invention relates to a novel method for converting bleaching filtrates into a pure liquid which can be reused, preferably in the bleaching plant.

Solution

In the method according to the invention, the bleaching filtrate is supplied to the cooling system in a plant for gasifying and/or partially or completely combusting cellulose spent liquors, with the water content of the filtrate converting wholly or partially into steam. The formed steam is subsequently cooled, after which thereby formed condensate is drawn off for direct or indirect return to the process water system of the pulp mill. The principle of such a combustion plant, in which the method can be applied, is well known to the person skilled in the art and is described in more detail, for example, in US 4.808.264 and SE-C- 464 921.

The hot combustion gas which develops during the combustion is cooled rapidly by direct contact with

an aqueous cooling liquid. It is already well known that the cooling liquid can consist of water, condensate, green liquor or weak liquor, or combinations of these. However, it is not known that the cooling liquid could consist of filtrates from bleaching plants, something which has shown to be advantageous, inter alia with regard to the water balance and evaporation requirements of the modern mill. In the following, the plant to which the bleaching filtrate or Q filtrate is supplied will be designated as a gasification plant, even though the invention also can be applied in conjunction with complete or overstoichiometric combustion of cellulose spent liquors, that is with a greater oxygen supply than that which is needed stoichiometrically for the combustion. An example of a cooling system in such a plant is described in Swedish Patent Application SE-A- 9001957. While the bleaching filtrate in question can be subjected to different treatments, for example in accordance with points 1-3 above, before being supplied to the gasification plant, it can also be supplied to the cooling system of the gasifier directly and without previous treatment.

Description of the figures

The method will be described below on the basis of

Figure 1, which shows a preferred embodiment of a gasification plant for applying the method according to the invention, and

Figure 2, which shows the integration of a gasification plant according to Figure 1 into a pulp mill.

Black liquor (17) , or other cellulose spent liquor, is supplied to a pressure vessel containing a ceramically lined gasification reactor (1) . The reactor is also provided with an inlet (not shown) for oxygen

or oxygen-containing gas, and a burner. The reactor includes a liquid bath or quench (2) for collecting green liquor which is formed when the mixture of gas and smelt, resulting from the gasification, from the respective reactor is cooled by direct contact with the aqueous " cooling liquid. In addition to the quench, the cooling system of the gasification plant also consist of a venturi scrubber (3) with appurtenant liquid separator (4) and a gas treatment tower (5) . According to the invention, bleaching filtrate (6) , preferably from a chelating stage, is supplied to the cooling system, in this preferred case the quench (2) . When the filtrate liquid and the hot combustion gases from the oxidation process come into contact, water evaporates to generate steam, which steam is drawn off from the reactor and the quench together with the partially cooled combustion gas (7) . The inorganic content in the remaining part of the filtrate liquid is mixed with the green liquor in the cooling system and is drawn off in the form of a liquid (16) . Any organic compounds which are present in the filtrate liquid are likewise separated off together with drawn-off liquid (16) in the form of green liquor. In this way, a purification of the bleaching filtrate takes place, as a result of which undesirable compounds which have been separated off end up in the green liquor. The latter is then purified in a green-liquor filter.

The temperature of the combustion gas (7) which is drawn off from the reactor and the quench corresponds in the main to the boiling point of the drawn-off liquid (16) at the pressure prevailing in the system.

After the quench stage, the combustion gas (7) is subjected to further washing in a venturi scrubber (3) and liquid (8) is then separated off in a liquid separator (4) . This liquid which has been separated off is returned to the venturi scrubber. Due to the fact that a certain quantity of the liquid (9) is bled off from this recirculation and a corresponding quantity of

purer liquid (10) is supplied, undesirable compounds do not become concentrated in the liquid.

When the gas (11) is cooled down still further, preferably by means of indirect cooling with water (13) and weak liquor (14) , in a gas-treatment tower (5) , the steam in the combustion gas condenses. This results in a weakly alkaline liquid (12) . This liquid is preferably stripped with steam in order to drive off sulfide and is then returned to the bleaching plant . The combustible gas (15) is transferred directly or indirectly to a gas boiler for final oxidation.

The quantity of bleaching effluent filtrate which can be supplied to the cooling system of a gasification plant in this manner varies with the capacity of the plant, the gas flow, the gas composition, the gas pressure and the gas temperature at the outlet of the reactor prior to cooling, and also with the desired strength and composition of the drawn- off liquid. The invention is elucidated in the following examples.

Examples

A plant for gasifying black liquor, described in more detail in US 4.808.264, is supplied with 500 tons of black liquor, calculated as dry substance, per 24 h.

Reactor temperature 950°C Reactor pressure 0.5 atmosphere above atmospheric pressure The hot combustion gas which evolves under these conditions has the following composition: CO 10.7%

C0 ₂ 11.0%

H ₂ 0 17.8%

H ₂ 11.8%

N ₂ 47.3%

H ₂ S 0 . 1 %

CH ₄ 1 . 3 %

The smelt which is generated in the reactor has the following principal composition

Na ₂ C0 ₃ 70.0%

NaOH 6.6%

Na ₂ S 22.2%

NaCl 1.1% The hot combustion gas is contacted by Q-stage filtrate from a TCF bleaching plant in a cooling device and dissolver, for example of the type which is described in more detail in SE-A-9001957. The quantity of filtrate which is supplied to the system corresponds to 74 m ³ /hour. The filtrate contains 1.6% dry substance in the form, inter alia, of undissolved organic substance, sodium compounds and transition metals such as manganese. If desired, the pH of the filtrate can be adjusted to 6-8. A green liquor flow of 48 m ³ /hour, having a concentration of 160 grams/litre (calculated as NaOH) , is drawn off from the quench. With the combustion gas, which is now saturated with steam, accompanies a water flow corresponding to 46 m ³ /hour. The temperature of the combustion gas drawn off from the reactor and the cooling system essentially corresponds to the boiling point of the drawn-off liquid at the pressure prevailing in the system, i.e. approximately 80-90°C in this example. When the gas is cooled down to about 40°C,

44 m ³ /hour of weakly alkaline liquid condenses out. This liquid is stripped with steam in order to drive off sulphide and is returned to the TCF bleaching plant. The green liquor stream is filtered in a green liquor filter and conveyed to causticization. The combustible gas is transferred directly or indirectly to a gas boiler for final oxidation.

Figure 2 shows how a gasification plant (22) which operates in accordance with the present method

can be integrated with a fibre line, including bleaching plant (20) , and recovery boiler (21) together with dissolving tank (23), in a pulp mill. In this case, black liquor (17) is apportioned between the recovery boiler and the gasification plant, both of which produce green liquor (16) . Bleaching effluent (6) is also supplied to the gasification plant, which effluent, according to the invention, undergoes an evaporation. Condensate (12) from this evaporation is preferably returned to the bleaching plant (20) for use as washing liquid in the latter.

The substances which remain in liquid phase during the evaporation end up in the green liquor and are then separated from the latter in a green liquor filter (24) .

Areas of application and alternative embodiments of the invention

The partial oxidation, or alternatively the combustion, of the cellulose spent liquor can be carried out at temperatures of between 700-1200°C and at pressures of from 0.1 MPa to 10 MPa.

Even if it is preferred to add the Q filtrate to the cooling system at a point at which direct contact is achieved between the hot combustion gas and the filtrate liquid, it is also possible, according to the invention, to add the filtrate liquid to any point in the cooling system. The invention can be put into practice in conjunction with several variants of gasification and combustion of cellulose spent liquors. For example, the cellulose spent liquor per se can be a sulphite spent liquor, soda spent liquor or bleaching effluent liquor. The stoichiometry during the oxidation in the reactor can vary from 0.3 to overstoichiometric combustion. If the oxidation process in the reactor takes place at a pressure exceeding 0.3 MPa, the steam partial pressure and energy content of the combustion gas can be

utilized for evaporating a further quantity of filtrate, thereby achieving multi-stage economy in the evaporation. If a more dilute green liquor is acceptable (for example 75 g/1, calculated as NaOH) , a correspondingly larger quantity of Q filtrate can be supplied to the combustion plant.

The Q filtrate specified in the description is not limited to filtrate from chelating stages, and any bleaching filtrate whatsoever, or any other water- containing process stream, can be purified and evaporated by means of the present invention.

The present invention can be applied in mill systems using either TCF or ECF bleaching, but is especially applicable in conjunction with TCF bleaching - for example in association with bleaching plants having the following bleaching sequences: Q (OP) (ZQ) (PO) Q (OP) Pa (PO) Q (PO) Q Z P

Q O Q 0 PO Q P (ZQ) (PO) where Q is an acidic stage to which complexing agents are added 0 oxygen stage

(OP) peroxide-fortified oxygen stage Z ozone stage Pa peracetic acid stage (PO) pressurized peroxide stage The invention is not limited to embodiments specified in the description, and can be varied within the scope of the subsequent patent claims.