JPS5943600B2 - 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, the pH is 10.
The above alkaline aqueous solutions are used.

It is obvious 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 continue to maintain the hydrogen sulfide absorption effect, 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 alkaline solution with a pH 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 pH is 9 or less, carbon dioxide gas will not be absorbed, but hydrogen sulfide absorption will be 50 to 40% or less.

However, in this method, trace amounts of iron salts, nickel salts, manganese salts such as iron sulfate, nickel sulfate, manganese sulfate, etc. are added as a catalyst either singly or in a mixture of two or more of the above salts. By simply doing this, it is possible to selectively absorb hydrogen sulfide and sulfur dioxide gas without the need for large oxidation equipment and in a pH range of 10 or less without being affected by carbon dioxide gas.

Furthermore, the present invention is characterized in that a chemical solution generated in the paper mill chemical recovery process is used as an alkaline solution to replenish the alkali consumed by absorption of hydrogen sulfide and sulfur dioxide gas.

The alkaline aqueous solution used as the absorbing liquid absorbs carbon dioxide gas in a high pH region, and as a result, the pH value decreases.
Eventually, when the liquid composition reaches equilibrium with the partial pressure of carbon dioxide in the gas, no further absorption of carbon dioxide occurs, and the p due to carbon dioxide gas increases.
No decrease in H is observed.

This tendency for pH to stop decreasing begins at pH 10 or lower, and p
It will settle down around H9.

Further, when the pH 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, the pH range is preferably 8.3 to pH 1.0.0.

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 20°C to 95°C, and the absorption effect of hydrogen sulfide and sulfur dioxide gas was good.

By adding the catalyst under the above pH 1 concentration and temperature conditions, a high effect of absorbing hydrogen sulfide and sulfur dioxide gas was obtained.

Various metal salts as catalysts can be used in small amounts (307n9/l)
Sufficient absorption effect can be achieved even below.

Iron salts, manganese salts, nickel salts 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, manganese carbonate, manganese phosphate and nickel sulfate, nickel chloride, nickel nitrate, nickel hydroxide, nickel carbonate, nickel phosphate, nickel acetate Compounds of iron, manganese, nickel, etc. can be used.

The present invention aims to efficiently absorb hydrogen sulfide and sulfur dioxide gas under the above conditions, and to use the alkali generated during the chemical recovery process in paper mills as a replenishing alkali for the alkali consumed by the absorption of hydrogen sulfide and sulfur dioxide gas. Hydrogen sulfide and sulfur dioxide gases that escape into the exhaust gas generated during the paper mill chemical recovery process, such as a recovery boiler, are absorbed using an alkaline aqueous solution, such as green liquor, white liquor, or weak liquor, and the absorbed liquid is 1
This is an improved method for recovering chemicals by extracting a portion and returning it to the chemical recovery process to effectively recover the sulfur content.

Green liquor is a liquid obtained by dissolving the molten material after burning black liquor in a chemical recovery boiler with water or weak liquid.

White liquor is a liquid obtained by clarifying green liquor produced in the chemical recovery process at a kraft pulp manufacturing plant, and causticizing the clarified liquor in a causticizing process.

Weak liquid is the liquid after washing lime mud containing white liquor,
It is a diluted white liquor.

The -ψl of the compositions of green liquor, white liquor, and weak liquor produced in a kraft pulp manufacturing plant are shown below.

In addition, chemical solutions generated in sulfite pulp manufacturing plants include green liquor, which is a mixture of sodium sulfide and soda carbonate, and carbonated solution of green liquor, as well as soda carbonate containing a small amount of sulfide, etc., in various chemical recovery methods. There is a solution.

An example of those chemical solutions is shown below. Table -■ -fIl of the composition of green liquor is shown.

Na2Co3 (mol/l) 0.
99Na2S (mol/-e)
0.91Na2so4(mol/-e)
0.19Na2 S203 (mol/
l) 0.07Na2So3 (mol
/l) 0.03 Table-■ An example of a carbonated green liquor is shown.

Na2CO3 (mol/l) 1°4
ONa2S (mo1/l) 0.55N
a2SO4 (mol/l) 0.08
Na2 S20S2O5 (/l) 0.
05Na2SO3 (mol/l)
0.02 Table - ■ An example of a soda carbonate solution containing a small amount of sulfide is shown.

Na2CO3 (mol/l) 1.8
0Na2S (mol/A) 0.
14Na 2 SO4 (mol/l)
0.04Na2 s2o3 (mol/l')
0.02Na2so3 (rnol/
1. ) 0.01 As shown in Table ■ and Tables ■ to ■, when the chemical solutions in the chemical recovery process are used as alkali supplies, the sodium sulfide (Na
2S) enters the absorption liquid, the sulfide concentration in the absorption liquid increases, and finally hydrogen sulfide is generated from the absorption liquid.

However, in this case, the amount of chemical solution supplied to the absorption solution is small (
When the amount of sulfide flowing into the absorption liquid is small, the sulfide in the liquid is quickly oxidized by the oxygen in the gas and the catalysts of iron, manganese, and nickel while the absorption liquid passes through the absorption tower. Since the sulfide in the liquid is treated, no hydrogen sulfide is generated due to the supply of a chemical solution containing sulfide, and the effect of absorbing hydrogen sulfide and sulfur dioxide gas in the gas continues.

In addition, when the amount of chemical solution containing sulfide is supplied to the absorption liquid, the amount of oxygen in the gas alone cannot treat the sulfide in the absorption liquid, and sulfides of iron, manganese, and nickel are generated. Hydrogen sulfide is generated from the liquid, and the absorption effect of hydrogen sulfide in the gas deteriorates rapidly.

Even in this state, the absorption effect of sulfur dioxide gas remains unchanged (continues) as long as the pH maintains the above-mentioned conditions.

Even if a large amount of chemical solution containing sulfide is supplied to the absorption liquid and hydrogen sulfide is generated from the absorption liquid and the hydrogen sulfide absorption effect in the gas decreases, it is necessary to increase the oxygen concentration in the gas or
Sulfides in the liquid can be treated by simple air oxidation of the absorption liquid.

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 larger, and conversely, air will promote the generation of hydrogen sulfide from the absorption liquid. Become.

When a large amount of chemical liquid containing sulfide is supplied to the absorption liquid, the sodium sulfide in the chemical liquid is converted to sodium thiosulfate or sodium sulfate (sodium sulfate) by air oxidation of the absorption liquid.
Hydrogen sulfide in the gas is absorbed by the absorption liquid and turns into sodium thiosulfate or mirabilite (sodium sulfate), and sulfur dioxide gas turns into sodium sulfite or mirabilite.

These components are extracted from the process together with soda carbonate and acidic soda carbonate as an extraction liquid.

It is then used in the chemical recovery process. Although the present invention can also be applied to the chemical recovery process in sulfite pulp production, an example of the chemical recovery process in a kraft pulp manufacturing factory 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 are treated with alkaline cooking liquor (white liquor) in the cooking process to obtain residue and pulp.

After completion of cooking, the pulp and cooking liquor are blown into a blow tank and then sent to a washing process where they are separated into black liquor and pulp.

The separated pulp is sent to the next process via a screen.

On the other hand, since the black liquor is reused as a medicine after going through a concentration process, a recovery boiler burns the combustion components in the black liquor, removes the organic components, and separates and recovers the effective medicine.

The molten material obtained in the recovery boiler is called smelt and is dissolved in weak liquid to become green liquor.

After the crude green liquor containing undissolved residues is removed in a clarifier, the clarified green liquor is sent to a causticizing process.

In the causticizing process, the clear green liquor is mixed with quicklime and stirred to cause a causticizing reaction, where the soda carbonate in the green liquor turns into caustic soda and becomes white liquor.

After clarification, the white liquor is used again as cooking liquor.

In addition, quicklime becomes lime through a causticizing reaction in the causticizing process, and this is separated from the white liquor, washed, and then burned in a kiln to become quicklime and used again in the causticizing process.

The washed lime mud containing white liquor is called weak liquor and is used to dissolve smelt.

In this way, the chemical recovery process is mostly a closed system.

However, in reality, there is a loss of sulfur that escapes from recovery boilers as hydrogen sulfide and sulfur dioxide gas.

This method recovers the sulfur content of and does not disrupt 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 recovery boiler exhaust gas is guided to the absorption tower by a fan, and the absorption liquid is sent to the top of the absorption tower by a pump, the gas and liquid are brought into countercurrent contact, and the hydrogen sulfide and sulfur dioxide gas in the exhaust gas is absorbed into the absorption liquid.

Green liquor or white liquor or weak liquor is supplied to the circulation tank as the alkaline supply.

The circulation tank is aerated with air.

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.
It can be returned to the recovery boiler process, causticizing process, etc.

However, desirably, when green liquor is used to supply alkali, it is rational from various points of view to return the green liquor to the place from which it was taken out, and to return the white liquor and weak liquor to the place from which they were taken out as well.

In addition, sodium thiosulfate, mirabilite, and sodium sulfite in the returned liquid are treated in the recovery boiler step during the chemical recovery process.
Since it becomes soda sulfide and soda carbonate, it does not accumulate in the system.

Examples will be described below.

Example 1 When hydrogen sulfide and sulfur dioxide gas are absorbed and removed, green liquor is used as the alkaline supply liquid, an alkaline aqueous solution containing nickel sulfate as a catalyst is used in the absorption liquid, and the absorption liquid tank is aerated with air. An example is shown below.

The green liquor was supplied to the absorption liquid circulation tank, but no hydrogen sulfide was generated from the absorption liquid tank, and good results were obtained in removing hydrogen sulfide and sulfur dioxide gas from the exhaust gas, as shown in Figure 1. It was done.

The composition of the extracted absorption liquid in Example 1 is shown below.

Extracted and returned liquid Na2CO30, 238 mol/1 NaHC03o, 869 〃 Na2S2O30, 3601! Na2SO30,397〃 Na2SO40,010tt Liquid volume 5.4 l/H Example 2 When absorbing and removing hydrogen sulfide and sulfur dioxide gas, green liquor was used as the alkali supply liquid, and ferrous sulfate and manganese sulfate were used as catalysts in the absorption liquid. An example will be shown in which an alkaline aqueous solution to which was added at the same time was used, and the absorption liquid tank was aerated with air.

No generation of hydrogen sulfide was observed from the absorption liquid circulation tank due to green liquor supply.

Furthermore, the effect of removing hydrogen sulfide and sulfur dioxide gas in exhaust gas is shown in Figure 2.

The composition of the extracted absorption liquid in Example 2 is shown below.

Extraction and return liquid Na COO, 236rnl/1 3 NaHCo 0.864 〃Na2S2O
30,421tt NaSo 0.331 l! 3 Na SO'0.010 // 4 Liquid amount 8. I J/H As the absorption tower used in Examples 1 and 2, a packed tower was used.

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

Example 3 When hydrogen sulfide and sulfur dioxide gas are absorbed and removed, a weak liquid is used as the alkaline supply liquid, an alkaline aqueous solution containing manganese sulfate as a catalyst is used in the absorption liquid, and the absorption liquid tank is aerated with air. An example is shown below.

The weak liquid was supplied to the absorption liquid circulation tank, but there was no generation of hydrogen sulfide from the absorption liquid tank.

In this example, a 6-stage self-plate tower was used as the absorption tower.
The removal effect of H2S and SO□ is good (, H2S during 24-hour operation
The 2S removal rate was 90%, and the SO2 removal rate was about 95%.

The composition of the extracted absorption liquid in Example 3 is shown below.

Extraction and return liquid Na2CO30,102Cmol/A')Na2CO3
o, 612 Crnol/-e”JNa2S
203 0.323 [mol/l] Na2SO30,
235 [mol/l] Na2SO40,125Cmol/Al Liquid volume 5 1/H) Example 4 When absorbing and removing hydrogen sulfide and sulfur dioxide gas, a weak liquid was used as the alkaline supply liquid, and ferric chloride was added as a catalyst to the absorption liquid. An example will be shown in which an aqueous alkali solution containing .

The weak liquid was supplied to the absorption liquid circulation tank, but no hydrogen sulfide was generated from the absorption liquid tank.

In this example, a six-stage perforated plate base was used as the absorption tower.
The removal effect of H2S and S02 is good, and H2S and S02 are removed during 24-hour operation.
The 2S removal rate was 90%, and the SO2 removal rate was about 95%.

The composition of the extracted absorption liquid in Example 4 is shown below.

Extraction and return liquid Na2CO30,095Cmol/l ]NaHC0
3o, 552 (mol/l: )Na2S,,
03 0.340 Cmol/l'JNa2
5o3o, 103 Cmol/l) Na2so
4 o, 260 Cmol/l) Example 5 When absorbing and removing hydrogen sulfide and sulfur dioxide gas, a weak liquid was used as the alkaline supply liquid, an alkaline aqueous solution to which ferric sulfate was added as a catalyst was used in the absorption liquid, and An example is shown in which air aeration is performed in the absorption liquid tank.

The weak liquid was supplied to the absorption liquid circulation tank, but no hydrogen sulfide was generated from the absorption liquid tank.

In this example, a three-stage perforated plate base was used as the absorption tower.
The H2S removal rate at this time was as poor as 60-70%.

So, if I increase the perforation plate to 6 levels and keep it the same as before, H2S1
The removal effect of S02 is also improved, 90% with H2S, S02
About 95% was obtained.

[Brief explanation of the drawing]

Figure 1 shows the H2S and SO2 absorption and removal rates in Example 1, Figure 2 shows the H2S and SO2 removal rates in Example 2, and Figure 3 shows the H2S and SO2 removal rates in Example 1 and 2. A schematic flow sheet is shown. FIG. 4 is a diagram of a kraft pulp 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 ... Green liquid, 8 ... Green liquid tank, 9
...Air, 10...White liquid, 11...
...Weak liquid, 12...Blower, 13...
・Catalyst, 14... Catalyst tank, 15... Raw material chips, 16... Cooking process, 17...
・Cleaning process, 18...Screen, 19...
... Concentration step, 20 ... Recovery boiler 1 step,
21... Causticizing process, 22... Kiln, 23... Electrostatic precipitator, 24... Chimney, 25... Unbleached pulp, 26...black liquor, 27...smelt, 28...lime (CaCO3), 29...quicklime (Cab),
A: Return of liquid after absorption when green liquor is used as alkali supply, B: Return of liquid after absorption when white liquor is used as alkali supply, C: After absorption when weak liquid is used as alkali supply. Return of liquid.

Claims (1)

[Claims] 1. In the chemical recovery process at a paper mill, the alkaline chemical solution generated during the chemical recovery process is used to absorb hydrogen sulfide and sulfur dioxide gas that escapes into the exhaust gas generated from the paper mill's chemical recovery process, such as a recovery boiler, and remove the sulfur. In the method of recovering manganese and nickel salts, or two of each of iron, manganese and nickel salts,
An improvement in the chemical recovery process in a paper mill, characterized in that the above-mentioned alkaline aqueous solution to which a mixture of at least 1 species is added as a catalyst is used as an absorption liquid in a pH range of 8.3 to 10.0 to absorb hydrogen sulfide and sulfur dioxide gas. Method.