FI89518B - Method for recovery of sulphur and alkalis - Google Patents

Abstract

The present invention concerns a method for recovery of alkali and sulphur in a pulp factory from a solution containing sulphide or bisulphide and the smokestack gases which arise in connection with the chemical recovery processes of the pulp factory. Green liquor or another sulphur-containing solution is carbonated with carbon dioxide, whereupon alkaline bicarbonate and hydrogen sulphide is formed. The hydrogen sulphide escaping with the carbon dioxide is separated as elemental sulphur by letting the hydrogen sulphide react with sulphur dioxide and the carbon dioxide is recirculated to the carbonation process for the green liquor and the bicarbonate formed is taken away as a carbonate/bicarbonate solution. The hydrogen sulphide is made to react with sulphur dioxide which has been recovered, for example, from the smoke gases of a sludge furnace. The carbon dioxide is removed with steam from the resulting carbonate/bicarbonate solution, thus forming a carbonate solution which is treated to recover the alkali. <IMAGE>

Description

8951 8

METHOD FOR THE RECOVERY OF SULFUR AND ALKALI

The present invention relates to a process for the artistic recovery of alkali and sulfur from a solution containing sodium sulphide or bisulphide in a pulp mill and from the flue gases generated during the chemical recovery processes of the pulp mill.

10 Factories today strive to achieve ever lower chemical emissions. In pulp mills, for example, this means ever lower emissions of sulfur, which means that sulfur must be recovered and returned to the mill for chemical recycling or other uses where it does not harm the environment.

15 In this case, however, care must be taken to ensure that the chemicals are returned and disposed of in the correct way so that the chemical ratios in the plant, in particular the sodium-sulfur ratio, are kept in balance.

20 In a pulp mill, a solution containing alkali sulfide and sodium carbonate is obtained e.g. by burning the cellulose broth waste liquor under reducing conditions and the resulting chemical-containing melt is dissolved into green liquor. Several methods are known for separating sulfur from an alkali sulfide solution by carbonizing the solution to regenerate cooking chemicals used in the manufacture of cellulose. In the so-called • ♦ ·. · 1 Stora chemical recovery process for sulphite soup • · »: sulphide liquor is recovered from green liquor solutions • ·: in a process in which excess carbon dioxide gas is *: 2:30 reacted with a solution to form a gaseous mixture containing excess carbonide and hydrogen sulfide to simultaneously obtain a solution that is substantially free of sulfide sulfur. This solution • 1 · * · P1065 / T3 may react with alkali bisulphite or sulfur dioxide, • · 35 to release carbon dioxide to give an alkali metal sulphide 2 • · .1 ·; a phytic solution that can be used as a cooking liquor. Hydrogen dioxide • · · · 2 39518 Hydrogen sulphide in the binder gas may react with sulfur dioxide to form elemental sulfur (Claus reactor). The sulfur is burned in a separate furnace in the presence of air to produce sulfur dioxide, which is used in the reaction with hydrogen sulphide 5 and also in the reaction with the waste solution after the solution has been substantially freed of sulphide sulfur using carbon dioxide.

In pulp mills, sulfur can be released into the environment mainly with flue gases, which means that the flue gases must be washed to bind the sulfur.

It is an object of the present invention to provide a simpler and more economical way of recovering chemicals from a pulp mill 15 in an effort to minimize mill emissions and maintain chemical ratios.

In the process according to the invention - green liquor or other sulphide-containing solution is carbonated with 20 carbon dioxide to form time bicarbonate and hydrogen sulphide, - hydrogen sulphide leaving with carbon dioxide is separated off is removed as a carbonate / bicarbonate solution, and it is essential for the process that - hydrogen sulphide is reacted with sulfur dioxide recovered from the flue gases of the pulp mill, and - carbon dioxide is removed from the carbonate / bicarbonate solution obtained by steam to form a carbonate solution, to recover alkali.

*: 35 As the sulfur dioxide required in the Claus reactor is a by-product of the process, neither a separate sulfur incinerator nor P1065 / T3 3 8 9 5 'is required. 8 associated waste heat boilers as in the known Stora process. In this way, the sulfur dioxide recovered in the flue gas cleaning can also be utilized. The pure sodium carbonate solution can be causticized to sodium hydroxide, which can be used, for example, in pulp bleaching and thus reduces the need for make-up chemicals.

With the method according to the present invention, the Na and S balance of the entire pulp mill can be controlled so that it is not necessary to take chemicals out of the rest of the process.

The invention will be described in more detail with reference to the accompanying figure, which shows an apparatus for carrying out the method according to the present invention.

15

The clarified sulfide-containing green liquor is passed at optimum density, about 130 g Na 2 O / L (TTA), to the top of reaction tower 2 for carbonation. The reaction tower is, for example, a packed column or a spacer column. The green liquor is carbonated with 20 carbon dioxide to form bicarbonate and hydrogen sulfide. Carbonation can lead to only a part (for example · * ·. 20-25%) of the plant's green liquor stream, so that the process does not interfere with the actual production of white liquor. Alternatively, the entire green liquor stream can be treated in this way.

25 ··· Hydrogen sulphide exits with carbon dioxide from the top of tower 2 along compound 3 to a catalytic contact reactor, such as • · • ’Claus reactor 4, where it is converted to elemental sulfur in the presence of sulfur dioxide. Elemental sulfur 6 is obtained from the condenser 5 below the reactor, from which part 7 can be removed from the process or part 8 can be recycled. pulp for cooking by mixing sulfur with white liquor, thereby; sodium polysulfide is formed, which improves the yield of the soup.

35 Carbon dioxide 9 is recycled back to the reaction tower 2 for carbonation of the green liquor.

P1065 / T3 4 o 9 5. 8

Carbon dioxide is removed from the bicarbonate / carbonate solution formed in the carbonation by means of steam 10, which is fed from the bottom of the reaction tower, whereby the bicarbonate 5 is converted into carbonate. The substantially pure carbonate solution 11 is removed from the bottom of the tower 2.

Removal of the carbonate solution from the reaction causes a carbon dioxide divide, which is covered by feeding bicarbonate 13 to the bicarbonate zone of the reaction tower. The bicarbonate is obtained by recycling part 12 of the carbonate solution to an absorber 14, in which carbon dioxide is absorbed into the carbonate solution from a suitable source, for example lime furnace flue gases 15. The absorber 14 is, for example, a packing column or bottom valve column. The absorbed carbon dioxide is removed from the solution when steam is fed to the bicarbonate zone. Such an embodiment makes it possible to introduce carbon dioxide into the system as required by adjusting the degree of carbon dioxide absorption and removal into and out of the circulation and by adjusting the recirculation flow.

The remaining 16 of the sodium carbonate solution is passed to causticization. Burnt lime 17 is added to it, whereupon it goes out and at the same time the carbonate solution can be causticized.

* · · '·' * 25 The calcium carbonate 19 formed from the causticized solution is filtered off in a filter 18 and passed to a leachate. The resulting filtrate 20 is pure sodium hydroxide; solution of Al. For example, sodium hydroxide can be used in bleaching to replace pure NaOH purchased elsewhere.

·: ··: 30

The sulfur dioxide 21 required in the Claus reactor 4 is obtained. *. sulfur dioxide-containing flue gases from a pulp mill, such as a flue gas from a lime kiln or a separate flue gas combustion reactor. Most preferably, SO2 is recovered according to FI-patent application 915672. The sulfur-containing flue gas 22 is contacted with the sodium sulfite-containing P1065 / T3 5,395.8 scrubber 31 in the absorber 23, whereby sulfur dioxide is absorbed into the solution to form a solution containing sodium bisulfite. The absorption solution is regenerated by an evaporative crystallization method utilizing the thermocompressor-5 slurry evaporation technique. The bisulfite solution 24 is removed from the absorber and passed to an evaporation and crystallization device 25, which is preferably operated by a lamella evaporator operating on the landing stage principle, on the outer surface of which the liquid to be evaporated flows and the heating medium inside. 10 Concentration of the solution results in the thermal decomposition of sodium bisulphite to give water, sulfur dioxide and sodium sulphite which crystallises from solution. The solution 26 containing sodium sulfite crystals is removed from the evaporator for use as a washing solution. The formed water vapor / sulfur-15 dioxide mixture 27 is removed from the evaporator to a compressor 28, where the pressure and temperature of the steam are raised and the steam is returned to the evaporator for use as heating steam. The condensate 29 formed during the condensation of the steam is passed to a tank 30 and used to dissolve the sodium sulfite crystals 20, which solution is used in an absorber 23. The non-condensable sulfur dioxide gas 32 is passed to a condenser 33 outside the evaporator 25. Claus reactor.

»· 25:: In the process according to the invention, sulfur and alkali are recovered • ♦ * so that they can be led in the process to * where there is a need for the chemicals in question. In this way, an important Na / S balance can be adjusted in a controlled manner.

*: 30 P1065 / T3

Claims (6)

6 o 9 5, 8 Patent applications: A process for recovering alkali and sulfur from sulphide-containing green liquor or other sulphide-containing solution, which process comprises - carbonising liquor or other sulphide-containing solution with carbon dioxide to form alkali metal carbonate and hydrogen sulphide, reacting hydrogen sulphide with - the carbon dioxide is recycled back to the carbonation of the green liquor and the bicarbonate formed is discharged as a carbonate / bi carbonate solution, characterized in that - hydrogen sulphide is reacted with sulfur dioxide recovered from the flue gases of the pulp mill, and - the resulting carbonate / bicarbonate solution 20 account solutions treated to recover alkali. Process according to Claim 1, characterized in that the sulfur dioxide is separated from the flue gases by absorbing the sodium sulphite solution into the solution, the sodium bisite being formed. from the solution it · 'is recovered by regenerating the absorpt iol solution in a nailing and crystallization apparatus. • · · • · · # Process according to Claim 1 or 2, characterized in that the sulfur dioxide originates from a lime kiln or a separate reactor for the combustion of odorous gases. flue gases. Process according to Claim 1, characterized in that the carbonate solution is causticized with sodium hydroxide. Diksi. P1065 / T3 7 ό 9 58 Process according to Claim 1, characterized in that a part of the carbonate solution obtained is reacted with carbon dioxide from an external carbon dioxide source to form bicarbonate and the bicarbonate / carbonate solution thus obtained is decarbonated to carbonate the green liquor. Process according to Claim 5, characterized in that the carbonate solution is reacted with carbon dioxide in the flue gas of a lime kiln or recovery boiler. P1065 / T3 8 o 9 5. • · · • · · · · · · • • P1065 / T3 8 o 9 5. 8