FI104504B - Process for the purification of flue gases from the scent combustion plant of a sulphate pulp mill - Google Patents

Description

104504

Method for purifying the flue gases of a sulphate pulp mill odor gas plant

The invention relates to a process for purifying the flue gases of a sulphate pulp mill odor gas combustion plant 5 of the type described in the preamble of claim 1.

Significant amounts of gaseous sulfur compounds such as hydrogen sulfide and dimethylmercapane are produced as a by-product in the sulfur cycle of the sulfate pulp mill. In order to eliminate odor, these the odor gases are burned in a special waste heat boiler, resulting in carbon dioxide, water and sulfur dioxide. In order to minimize the sulfur emissions from the waste heat boiler, the sulfur dioxide in the post-boiler liquor scrubber is trapped with sodium hydroxide solution 15 and the resulting sodium sulfite and sodium sulfate solution is recycled to the mill's chemical circuit. Excess sulfur is removed from the chemical cycle, for example, as sodium sulfate dust from power filters, generally directly into the water system.

Sodium hydroxide, used as a neutralizing chemical, causes a large 20 batch run in the flue gas neutralization. It is estimated that at a mill producing 500,000 tonnes of pulp per year, the cost of chemicals, when used in the form of lye, is about FIM 15 million a year. This is equivalent to neutralizing about 500 kg of sulfur per hour. Sodium sulphate to be removed from the chemical cycle is water soluble, and the common way to remove it has been to dissolve it in water, which is further discharged to the waste water.

It is an object of the invention to provide a more economically viable method for purifying flue gases without the need for major changes in the process at the sulfate pulp mill 30. To accomplish this purpose, the method according to the invention is mainly known. .. be saddled with what is set forth in the characterizing part of the attached claim 1. The neutralizing agent used in the flue gas scrubber is an alkaline lime compound, which may be a normal 35 lime wet scrubber. The calcium compound formed in the reaction with the sulfur dioxide, i.e., calcium sulfite and / or calcium sulfate, is fed to the mill's wastewater treatment system. In this case, most of the 2,104,504 calcium compounds are soluble in the waste water and can be removed from the process easily and without significant additional investment in equipment.

The invention consists of two parts which together form an inventive whole:

The lye previously used in the sulphate pulp mill has been replaced by a lime compound. This is new in the case of sulphate pulp mills, as they have previously retained lye (NaOH), which has been suitably available in the mill's chemical cycle.

The calcium compounds produced during the purification are discharged into the waste-15 water system. When using lime compounds in flue gas purification technology, calcium sulfite and / or calcium sulfate have been separated and not derived from a large amount of wastewater, which also contains substances from other processes.

20

Subsequent dependent claims 2 to 6 disclose some preferred embodiments of the method. The calcium compound can be led to a pre-clarification of the fibrous waste water from the sulfate pulp mill, where it is removed with the slurry. Calcium sulfite, an immediate reaction product of sulfur dioxide 25 and lime compound, can be oxidized to a more water soluble calcium sulfate. As the lime compound, any inorganic alkaline calcium compound such as limestone, calcium carbonate (CaCO3), calcium oxide (CaO) or calcium hydroxide (Ca (OH) 2) otherwise obtained can be used.

30

The invention will now be described in more detail with reference to the accompanying drawings, in which Figure 1 is a flow diagram illustrating the purification of flue gases by the process of the invention, and Figure 2 shows a waste water treatment system according to the method of the invention.

104504 3

Figure 1 illustrates a process comprising a flue gas scrubber 1 to which sulfur dioxide-containing flue gases from a waste heat boiler are fed through a channel 2. In connection with the scrubber 1, there are normal wash water cycles 3, 5 in which the washing liquid is introduced into the upper part of the scrubber and recirculated from the lower part of the scrubber. The fresh washing solution is introduced into the scrubber via channel 4 and the used contaminated washing solution is discharged from the process via channel 5.

The process comprises a limestone dispenser 6 comprising a feed tank and a screw conveyor for feeding fine limestone into the mixing tank 7. Water is supplied to the mixing tank and the lime slurry obtained in a certain mixing ratio is fed to the washer 1.

15

In the scrubber, lime reacts with sulfur dioxide in a known manner to form calcium sulfite, some of which oxidizes to calcium sulfate depending on the oxygen content of the flue gases.

The resulting calcium sulphite / calcium sulphate slurry is discharged from the scrubber via conduit 5 to the mill's wastewater treatment system. Figure 2 shows schematically the locations where the slurry can be led. The sludge can be led to the pre-clarification 9 from the plant's fibrous wastewater 8, from where it passes to the sludge treatment 10. It is also possible to discharge the sludge 25 into the a is oxidized by the oxygen contained in the air supplied to the pool to readily soluble calcium sulfate or gypsum. The resulting gypsum can be separated in the effluent system in the after-treatment tank 12 after the aeration basin 11, from where it ends up in sludge treatment 10. Since calcium compounds are discharged into a large amount of sulphate pulp mill wastewater, it is also possible that most of the calcium sulphate Then it is dissolved in the purified water 13 which is discharged after the decontamination tank 12.

104504 4

Another possibility to oxidize calcium sulfite to calcium sulfate is to introduce air or oxygen gas into the lower part of the scrubber 1 along line 14 as shown in Figure 1. The calcium sulfate slurry thus obtained can be led to the same sites in the wastewater system as described above.

5

It can be estimated that the cost of neutralization, for example with limestone, at a mill producing 500,000 tonnes of pulp per year would be about FIM 4.5 million a year, thus saving about FIM 10 million per year. The large amounts of water in the sulphate pulp mill allow for the treatment of the calcium compounds generated by the neutralization, and no separate treatment plants are required, but they can be treated with existing equipment such as aerators, clarifiers and sludge presses. Also, the amount of calcium compounds coming from the scrubber does not affect the wastewater treatment system as the wastewater is neutralized with 15 other limes.

The invention is not limited only to the embodiments shown in the preceding schemes, but may be varied within the scope of the inventive idea of the claims. Instead of the 20 limestone fed into the mixing tank 7, another inorganic calcium compound can be used. Diluted lime-water solution can also be used as a washing liquid with a one-pass operation or a low recycling rate. In this case, the excess lime contained in the solution after the scrubber 1 can also act as a neutralizing agent in the mill's waste water treatment system. In addition, it is possible to use the alkaline lime compounds contained in the ash of the sulphate pulp mill bark boiler, in particular fly ash, as described above.

Claims (3)

104504 A process for purifying the flue gases of a sulfate pulp mill odorous combustion plant comprising cleaning the flue gases containing sulfur dioxide by introducing into the flue gas scrubber (1) a neutralizing agent which reacts with sulfur dioxide, characterized in that the alkaline ) from the flue gas scrubber (1) to the wastewater treatment system (9,10,11,12) of the same sulphate pulp mill, either to the pre-clarification (9) of the fibrous wastewater from which it is discharged with the sludge, or to the aeration Method according to claim 1, characterized in that 15 oxygen-containing gaseous media are introduced (14) into the flue gas scrubber (1) to oxidize the calcium sulfite to the calcium sulfate in the scrubber, whereupon the calcium compound is fed to either the fibrous waste water pre-clarification (9) or Process according to claim 1 or 2, characterized in that the alkaline lime compound is one or a combination of the following: limestone, CaCO 3, CaO, Ca (OH) 2. Claim. 104504