FI94444C - Method for balancing a pulp mill's sulfur / sodium ratio - Google Patents

Description

94444

Method for balancing the sulfur / sodium ratio of a pulp mill 5 In sulphate cooking, the resin acids, free fatty acids and triglycerides contained in the wood are saponified. The soaps formed during cooking form micelles that are transferred to the black liquor during pulp washing and rise in the storage tank to the surface of the black liquor, from where they are removed. The soap is collected 10 from the surface of mainly lean and intermediate black liquor tanks by overflow or decantation. The separated soap contains about 50% black liquor. It also contains lignin dissolved in black liquor.

15 Soap is traditionally broken into tall oil with sulfuric acid. Sodium sulphate is also formed. Sodium sulphide in black liquor forms hydrogen sulphide and lignin precipitates. Tall oil is separated from the reaction mixture and the remaining mother liquor is normally neutralized with lye, and returned to the evaporator as lean or intermediate black liquor. The H2S-containing gas is washed with lye to separate the sulfur.

The use of sulfuric acid in the column increases the sulfur load and sulfur emissions of the chemical cycle, and already leads to an excess of sulfur-25 in the chemical cycle. Excess sulfur in the plant will increase • as sulfur emissions need to be reduced as environmental regulations become more stringent.

Instead of pure sulfuric acid 30, waste sulfuric acids from other processes, such as Na 2 SO 4 -containing waste acids from various chlorine dioxide processes, can be used instead of pure sulfuric acid. There are also problems with such tall oil production. Corrosion of equipment increases due to the presence of chlorides. The adjustment of the peeling process is scrub-3b sheep, because the acid concentration does not always remain constant. In addition, the amount of waste acid and the separating soda can vary so that the soap / acid ratio does not always match, in which case it may be necessary to use * 2 94444 as pure sulfuric acid or to drive some of the acid to a recovery boiler.

Processes have been developed to partially or completely replace the 5 sulfuric acids used in the spinning process. Admittedly, they have hardly been exploited due to poor profitability. In the so-called Westwaco process, 50% of sulfuric acid can be replaced by carbon dioxide. However, a conventional sulfuric acid process is also required. The use of salt-10, acetic and formic acid instead of sulfuric acid has also been studied, but they have not proved to be profitable, e.g. because of the price of acids.

SE-A-386693 discloses a process for preparing tall oil 15 by columning soap with sulfur dioxide.

In this case, the formation of hydrogen sulfide can be considerably reduced, which is one of the problems of sulfuric acid digestion. SC 2 and soap are introduced into a sealed reaction vessel to form tall oil and a solution of sodium bisulfite and SC 2. The sulfur dioxide required in the process can be produced from the flue gases of black liquor combustion or from the flue gases of combustion of odorous gases. In this case, however, the plant is not self-sufficient in the sulfur dioxide required for columning, and gas produced from outside the plant may have to be used. As mentioned above, additional sulfur; however, is an unfavorable option.

It is an object of the present invention to provide a more economical method of crushing raw material 30 which balances the chemical cycle of the plant.

* Finnish patent 75615 discloses a method for heat treating black liquor, whereby the viscosity of the liquor is reduced and its treatment properties are improved. The temperature of the black liquor is raised above the cooking temperature, whereupon the high molecular weight lignin fraction is decomposed and a large amount of sulfur-containing gases is released from the liquor. Sulfur-containing gas contains e.g. dimethyl sulfide. In particular, these gases are utilized in the process of the present invention.

A method for balancing the sulfur / sodium ratio of a pulp mill, in which crude soap is separated from black liquor containing soap from wood extracts in cooking, which is columned in a manner known per se by reacting the soot with sulfur dioxide to form a reaction mixture from separated black liquor by heat treatment. The process is characterized in that at least a part of the sulfur dioxide required for the columning is produced from the sulfur dioxide-containing flue gases produced by the combustion of the sulfur-containing gases produced by the heat treatment of the black liquor.

An advantage of the method according to the present invention is that the required sulfur dioxide is introduced from inside the pulp mill. In this case, it is possible to carry out the columning without buying chemistry, which reduces the sulfur injection into the chemical cycle of the pulp mill, and thus the S / Na ratio is balanced.

In the heat treatment of black liquor, sulfur-containing gases can be produced in such abundance that the plant is substantially self-sufficient in terms of the SC> 2 required for the production of tall oil.

•

Of course, other sulfur dioxide-containing gases from the pulp mill can also be used in the process according to the invention, such as gases from the combustion of black liquor and the combustion of other odorous gases.

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 shows a method according to the invention for balancing the chemical cycle of a pulp mill 35, producing sulfur dioxide essentially from tallow, and Figure 4,94444.

The pulp 1 formed in the cooking of the wood is washed, whereby the separated soot is separated from the lean black liquor 2 only. The black liquor 4 is concentrated in a multi-stage evaporator, whereby It has been found that by pressure heating 10 black liquors at more than 200 ° C, 30-60% of the sulfur in the liquor can be removed as a gas rich in organic sulfur compounds such as dimethyl sulfide. The concentrated and heat-treated black liquor 5 is fired in a recovery boiler or the like to form a melt 15 6 for making cooking liquors.

The heat treatment of the black liquor can be performed before evaporation. If the black liquor is evaporated in a multi-stage evaporator, as is usually the case, the pressure heating 20 can be carried out at one or more points in the evaporator. The treatment can also take place after evaporation to improve the injectability of the black liquor into the recovery boiler. The treatment may be performed according to one or more options.

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The heat treatment of the black liquor is carried out by pressure heating at a temperature higher than 170 ° C, preferably 170-350 ° C. The treatment time depends on the temperature and the quality of the lye. Typically, the treatment time is 1 to 60 minutes to produce significant amounts of sulfur-containing gases.

The gas 7 generated by the heat treatment of the black liquor is further processed to produce sulfur dioxide. The gas is combusted and SO2 is recovered from the gas 8 thus obtained. This can be done by known methods, of which, however, the method disclosed in Finnish patent application 915672 is particularly preferred, in which sulfur dioxide is separated from the flue gases by absorbing it in a sodium sulphite lithium solution. The method is shown in Figure 2. The flue gas 8 containing sulfur dioxide is led to conventional venturi scrubbers 30 or an absorption tower. Liquid 31 containing sodium sulfite is introduced into the scrubbers. The purified gas passed through the two scrubbers 5 escapes through the chimney 32 into the environment.

10

The solution 36 to be regenerated is fed through the upper part of the evaporator 37 to the compartment 12. The heat transfer member 38 of the evaporator is formed by lamellae, on the outer surface of which the liquid to be evaporated flows and on the inside heating steam. Some of the solution at the bottom 15 of compartment 39 is returned to the top of compartment 39 and the rest is transported to compartment 40 where evaporation is continued. If necessary, in order to achieve a washing effect, the feed order can be changed and the solution to be regenerated can first be fed to the compartment 40.

20

The steam separated from the solution in the evaporator and the sulfur dioxide 41 formed are removed from the evaporator to the compressor 42 to increase the temperature and pressure. The steam mixture is fed to most of the equipment as heating steam from below-25. In this case, it encounters its own condensate, thus obtaining: a stripping effect. From here, the non-condensable steam goes to a small part of the equipment from above, and the condensate and non-condensable gases are removed from below. At the beginning of the process and if necessary, fresh steam 43 can be added.

30

The non-condensable sulfur dioxide gases 44 are led to a condenser 45 where most of the water vapor is condensed, and the remaining non-condensable gas 46 is concentrated sulfur dioxide. Its sulfur dioxide content can be adjusted by adjusting the condensing temperature 35 in the condenser 45. The recovered sulfur dioxide gas 46 can be used as mentioned above.

6 94444

The condensates 47 are led from the lamellae to a mixing tank 48, where a concentrated solution 49 containing sodium sulphite crystals and water are also introduced to dissolve the crystals and provide a new absorption solution which is recycled to flue gas purification.

The sulfur dioxide recovered by the process according to Figure 2 is led to the molding column. Sulfur 3 is mixed with sulfur dioxide and steam 12 in a mixer 11 to raise the temperature 10 to about 100-130 ° C. The MC mixer AHLMIXR (A. Ahlström Corporation) (FI patent application 870747), which is a combination of a fluidizing unit and a valve, is particularly suitable for carrying out the mixing. In its design, special attention has been paid to minimizing gaseous emissions. The reaction mixture 13 is passed to a pressurized reaction vessel 14 in which the main reaction is (1): RCOONa + SO 2 + H 2 O -> RCOOH + NaHSO 3 (1). A mixture 16 of tall oil and NaHSO 3 -containing mother liquor 16 is fed to a separator 17, such as a centrifuge or lamellar clarifier, where tall oil 18 is separated from the mother liquor 19 and the lignin precipitate. The mother liquor is neutralized, if necessary, with an alkaline solution 21. The neutralized mother liquor 20 is returned to the chemical circuit through a low-black liquor tank. If necessary, part of the mother water can be used for dry washing 22.

According to the invention, it is possible to advantageously utilize the sulfur compounds formed in the pulp mill inside the mill and thus reduce the sulfur emissions of the mill.

Claims (6)

94444 Claims; A method for balancing the sulfur / sodium ratio of a pulp mill, which method comprises separating crude soap from a black liquor containing soap formed in cooking 5 from wood extracts, which is columned in a manner known per se by reacting the soot with sulfur dioxide to form a reaction mixture from black liquor is heat-treated, characterized in that at least part of the sulfur dioxide required for columning is produced from sulfur dioxide-containing flue gases generated by the combustion of sulfur-containing gases from heat treatment of black liquor, whereby the plant is substantially self-sufficient in sulfur dioxide. A method according to claim 1, characterized in that sulfur dioxide produced from other sulfur dioxide-containing flue gases of the pulp mill 20 is additionally used in the columning. Process according to Claim 1 or 2, characterized in that the sulfur dioxide is separated from the flue gases by absorption. 25 of it into sodium sulfite solution to form a sodium bisulfite solution. Process according to Claim 3, characterized in that the sulfur dioxide is regenerated from the sodium bisulphite solution in an evaporative crystallizer. Process according to Claim 1, characterized in that the heat treatment of the black liquor is carried out before evaporation and / or at one or more points during evaporation and / or after evaporation. *: Process according to Claim 5, characterized in that the heat treatment of the iron liquor is carried out at a temperature higher than 170 ° C. 94444