JPS6015758B2 - Improved methods for chemical recovery in paper mills - Google Patents

Description

[Detailed Description of the Invention] Conventionally, in order to absorb hydrogen sulfide in gas, pHIO
The above alkaline aqueous solutions are used.

It is natural that this liquid can be used to absorb hydrogen sulfide, and it is clear that the absorption effect is sufficient. However, when used repeatedly as a hydrogen sulfide absorption liquid, sulfide accumulates in the liquid, and the hydrogen sulfide absorption effect gradually decreases. Therefore, in order to maintain the sulfide absorption effect continuously, it is necessary to remove sulfide from the liquid by some method. One possible method for this is to reduce the sulfide concentration in the liquid by oxidizing the sulfide in the air, but this naturally requires large oxidation equipment. Furthermore, when using an alkali solution with a ratio of 10 or more, a considerable amount of carbon dioxide gas contained in the exhaust gas consumes the alkali, which is not economically preferable.

If the boat is below 9, it will not absorb carbon dioxide gas, but
The absorption of hydrogen sulfide ends up being less than 50-40%. However, in this method, only a trace amount of iron salts such as iron sulfate and manganese sulfate, or manganese salts are added alone or in a mixture of the above salts as a catalyst, and large oxidation equipment is not required. Hydrogen sulfide and sulfur dioxide gas can be selectively absorbed in the range below pHIo without being affected by carbon dioxide gas. The alkaline aqueous solution used as the absorption liquid absorbs carbon dioxide gas in the region with a high axis, and as a result, the multiplier value decreases, but eventually, when the liquid composition reaches equilibrium with the partial pressure of carbon dioxide gas in the gas, no further absorption of carbon dioxide gas occurs. First, the deterioration of citrus flavor due to carbon dioxide gas is no longer observed.

The stop value of this group decrease starts from below pHIO, and boat 9
Calm before and after. Further, when the multiplication value becomes 8.3 or less, carbon dioxide gas starts to be generated from the liquid. Therefore, when using an alkaline aqueous solution as an absorbing liquid for absorbing and removing hydrogen sulfide and sulfur dioxide gas, it is desirable that the bait range be 8.3 to 10.0. Furthermore, the higher the concentration of the absorption liquid, the better the absorption effect of hydrogen sulfide and sulfur dioxide gas, but the absorption effect was found at 2% by weight or more in terms of sodium carbonate. Furthermore, the absorption liquid temperature was 20qo to 95°C, and the absorption effect of hydrogen sulfide and sulfur dioxide gas was good. By adding the catalyst under the above pH, concentration, and temperature conditions, a high absorption effect of hydrogen sulfide and sulfur dioxide gas was obtained.

Various metal salts used as catalysts may be used in small amounts, and even amounts of less than 309' can exhibit sufficient absorption effects. iron salt, manganese salt,
Examples include iron sulfate, iron chloride, iron nitrate, iron hydroxide, iron carbonate, iron phosphate, iron acetate, ammonium iron sulfate, iron chelate compounds, and manganese sulfate, manganese chloride, manganese nitrate, manganese hydroxide, carbonate. Iron and manganese compounds such as manganese and manganese phosphate can be used. If the concentration of hydrogen sulfide in the gas is low and the amount fixed as a compound in the absorption liquid is small, the oxygen in the gas and a catalyst such as iron or manganese will cause sulfidation in the liquid while the absorption liquid passes through the absorption tower. Since the substance is quickly oxidized and the sulfide in the liquid is treated, the effect of absorbing and removing hydrogen sulfide and sulfur dioxide gas in the gas continues. However, when the concentration of hydrogen sulfide in the gas increases, the amount of oxygen in the gas alone cannot treat the sulfide fixed in the absorption liquid, and sulfides such as iron and manganese are generated, causing hydrogen sulfide to be released from the absorption liquid. Occasionally, hydrogen sulfide in the gas may be generated and the absorption effect may deteriorate. Even in this state, the absorption effect of sulfur dioxide gas continues as long as the pH maintains the above-mentioned conditions. Even if the concentration of hydrogen sulfide in the gas increases, resulting in generation of hydrogen sulfide from the absorption liquid or a decrease in the hydrogen sulfide absorption effect in the gas, increase the oxygen concentration in the gas or simply remove the absorption liquid from the absorption liquid. Sulfides in the liquid can be treated by air oxidation. Of course, in this case as well, it is necessary for the catalyst to be present in the absorption liquid; without the catalyst, the oxidation device will become bulky, and conversely, air will promote the generation of hydrogen sulfide from the absorption liquid. . When efficiently absorbing hydrogen sulfide and sulfur dioxide gas under the above conditions, the present invention uses sodium hydroxide, sodium carbonate, and sodium sulfite as a replenishing alkali for the alkali consumed by the absorption of hydrogen sulfide and sulfur dioxide gas. Hydrogen sulfide and sulfur dioxide gas that escapes as exhaust gas generated from paper mill chemical recovery processes such as recovery pollers are absorbed using an alkaline aqueous solution such as . This is an improved method for recovering chemicals.

In order to maintain the chemical balance in the chemical process of paper mills, sodium and sulfur components are used to replenish the sodium lost in the sodium circulation system and the sulfur lost as hydrogen sulfide and sulfur dioxide gas in recovery boilers, etc. is supplied into the chemical recovery process.

However, by using the method of the present invention, the lost sodium is replenished with a sodium-containing alkaline aqueous solution such as sodium hydroxide, sodium carbonate, or sodium sulfite, and in the process of replenishing the sodium, the loss of sodium is replenished. By contacting gas containing hydrogen sulfide, sulfur dioxide gas, etc. generated from the paper mill chemical recovery process, the hydrogen sulfide, sulfur dioxide gas, etc. can be absorbed, and the absorbed liquid can be returned to the chemical recovery process. In order to suppress the loss of sulfur to the outside of the recovery process system, the amount of sulfur for replenishing the lost sulfur can be extremely reduced. Hydrogen sulfide in the gas is absorbed by the absorption liquid,
Converts to sodium thiosulfate or sodium sulfate,
Sulfur dioxide gas turns into sodium sulfite or sodium sulfate. These components, together with sodium carbonate and acidic sodium carbonate in the absorption liquid, are returned to the chemical recovery process as an extraction liquid. Although the present invention can be applied to various processes such as a chemical recovery process in the production of sulfite valves, an example of a chemical recovery process in a kraft pulp manufacturing plant incorporating the present invention will be described below with reference to the drawings.

In the chemical recovery process for producing kraft pulp, as shown in FIG. 4, raw material chips 15 are treated with an alkaline cooking liquor (white liquor) 80 in a steaming process 16 to obtain residual grooves and pulp.

After completion of cooking, the pulp and solution are passed through a blow tank and sent to a washing step 17, where they are separated into black liquor 26 and pulp. The separated pulp passes through a screen 18 and is sent to the next process. On the other hand, the black liquor 26 passes through the concentration step 19,
In order to reuse it as a chemical, the recovery polluter 20 burns the combustion components in the black liquor to remove organic components and separate and recover effective chemicals. At this time, in order to replenish the soda content lost in the soda circulation system, etc. and the sulfur content lost as hydrogen sulfide and sulfur dioxide gas in the recovery polluter, for example, trinitrate (Na2SQ) is used.
must be replenished. The melt obtained in the recovery boiler is called smelt 27 and is dissolved in weak liquor 31 to become green liquor 32. After removing the crude green liquor containing undissolved residue with a clarifier, the clarified green liquor is passed through the causticizing step 21.
sent to. In the causticizing step 21, the clarified green liquor is combined with quicklime 29 to cause a causticizing reaction, and the soda carbonate in the green liquor is changed to caustic soda, resulting in a white liquor 30. After the white liquor is clarified, it is used again as a cooking liquor. In addition, the quicklime 29 becomes lime 28 through a causticizing reaction in the causticizing process, and this is separated from the white liquor, washed, and then burned in the kiln 22 to become quicklime and used again in the tower forming process. The washed lime mud containing white liquor is called weak liquor 31 and is used to dissolve smelt. In this way, the chemical recovery process is almost a closed system. However, in reality, there is a loss of soda in the soda circulation system, and a loss of sulfur that escapes as hydrogen sulfide and sulfur dioxide gas from the recovery boiler. This method uses an aqueous solution of sodium hydroxide, sodium carbonate, or sodium sulfite to replenish this lost sodium,
In addition, when replenishing, the replenishment liquid is brought into contact with gas containing hydrogen sulfide and sulfur dioxide gas from a recovery boiler, etc., thereby absorbing and recovering the lost sulfur content that escapes out of the chemical recovery process system as hydrogen sulfide and sulfur dioxide gas. This method maintains the chemical balance in the chemical recovery process.

The process of the present invention is shown within the chain double-dashed line in FIG. First, the recovered boiler exhaust gas 1 is guided to the absorption tower 3 by a fan, and the absorption liquid 5 is sent to the top of the absorption tower by a pump, the gas and liquid are brought into countercurrent contact, and hydrogen sulfide and sulfur dioxide gas in the exhaust gas are absorbed into the absorption liquid. As an alkali supply, for example, a sodium carbonate solution 7 is supplied to the circulation tank 4. In the circulation tank, eel aeration is carried out using air 11. The catalyst is supplied from the catalyst tank to the circulation tank. In addition, a part of the absorption liquid is extracted and used in the chemical recovery process, that is, the black liquor concentration process 19 and the recovery boiler process 20.
, and can be returned to the causticizing step 21 and the like. Here, the case of returning to the concentration step is shown. Furthermore, sodium thiosulfate, E-nitrogen, and sodium sulfite in the returned liquid are treated in the recovery boiler process during the chemical recovery process and become sodium sulfide and soda carbonate, so they are not accumulated in the system. Examples will be described below.

Example 1 When absorbing hydrogen sulfide and sulfur dioxide gas, sodium carbonate was used as the alkaline supply liquid, an alkaline aqueous solution to which ferrous sulfate was added as a catalyst was used in the absorption liquid, and the absorption liquid tank was heated with air. An example is shown below.

Good results were obtained in removing hydrogen sulfide and sulfur dioxide gas from exhaust gas, as shown in Figure 1.

Example 2 When absorbing hydrogen sulfide and sulfur dioxide gas, sodium hydroxide was used as the alkaline supply liquid, and the absorption liquid was used as a catalyst.
An example will be shown in which an alkaline aqueous solution to which ferrous sulfate and manganese sulfate were added at the same time was used, and the absorption liquid tank was aerated with air.

Good results were obtained in removing hydrogen sulfide and sulfur dioxide gas from the exhaust gas, as shown in Figure 2. A packed column was used as the absorption column used in Examples 1 and 2.

A schematic flow sheet for Examples 1 and 2 is shown in FIG.

[Brief explanation of the drawing]

Figure 1 shows the removal rate of 2S and S02 in Example 1, Figure 2 shows the removal rate of S and SQ in Example 2, and Figure 3 shows the removal rate of Example 1 and its outline. A flow sheet is shown. FIG. 4 is a diagram of a craft valve manufacturing chemical recovery process incorporating the present invention. 1...Absorption tower inlet gas, 2...
Absorption tower outlet gas, 3... Absorption tower, 4... Absorption liquid circulation tank, 5... Absorption liquid, 6... Extraction liquid, 7... Sodium carbonate aqueous solution, 8... Water, 9 ... Sodium carbonate, 10... Alkali dissolution tank, 11... Air, 12...
・Blower, 13... Catalyst, 14... Catalyst tank, 15.
... Raw material chip, 16... Cooking process, 17... Washing and steaming process, 18... Screen, 16... Concentration process, 20.
... Recovery spoiler process, 21... Causticizing process, 22...
・Kiln, 23...Electric dust collector, 24...Chimney, 2
5... Exposed bulb, 26... Black liquor, 27... Smelt, 28... Lime (CaC03), 29... Quicklime (Ca○), 30... White liquor, 31... Weak liquid. Figure 1 Figure 2 Figure 3 Figure 4

Claims (1)

[Claims] 1. In the chemical recovery process at a paper mill, alkaline aqueous solutions such as sodium hydroxide, sodium carbonate, and sodium sulfite are supplied, and hydrogen sulfide escapes as exhaust gas generated from the paper mill's chemical recovery process, such as the recovery boiler process.
In the method of absorbing sulfur dioxide gas etc. and recovering the ion content, the above alkaline aqueous solution to which iron and manganese salts alone or a mixture thereof are added as a catalyst is used as an absorption liquid, and the pH is 8.3 to 10.0. Used within the range of
An improved method for recovering chemicals in a paper mill, characterized in that the liquid after absorbing hydrogen sulfide, sulfur dioxide gas, etc. is returned as is to the chemical recovery process.