JPS5930455B2 - Exhaust gas treatment method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for treating exhaust gas, and more specifically, it purifies exhaust gas using an absorption liquid containing an iron-based catalyst, and removes nitrogen compounds and sulfur that accumulate in the absorption liquid used for the treatment. The present invention relates to an exhaust gas treatment method capable of removing compounds, regenerating and recycling an absorption liquid, and recovering highly purified calcium sulfate.

In recent years, the removal of nitrogen oxides and sulfur oxides from exhaust gas has been raised as a major problem in terms of pollution control, and various wet purification methods for exhaust gas using absorption liquids containing iron-based catalysts have been proposed. .

Furthermore, methods for recycling and using these absorbed liquids are being considered, and it has been suggested that electrolytic reduction may be applied as one method.

However, in the known methods using these absorption liquids, the substance of the absorption mechanism is not clear, and when regenerating the absorption liquid and removing harmful substances transferred from exhaust gas to the absorption liquid, for example, a part of the absorption liquid is blown out or heat treatment,
Direct electrolytic oxidation, electrolytic reduction, etc. have been suggested, but the means necessary to realize them have not been fully clarified.

In an exhaust gas treatment method using an absorption liquid containing an iron-based catalyst, the present inventors have developed a better absorption liquid, effectively regenerated this absorption liquid, and removed sulfur compounds (especially sulfur) accumulated in the absorption liquid. We conducted extensive research on methods for separating and removing oxides) and/or nitrogen compounds.

As a result, a liquid containing (1) ferrous ions, (2) a predetermined amount of sulfite ions or hydrogen sulfite ions, and (2) alkanolamines is used as an absorption liquid, and the absorption liquid is treated by a diaphragm electrolytic reduction method using an anion exchange membrane. As a result, the iron-based catalyst is efficiently reduced and regenerated in the cathode section, and the sulfur compounds and/or nitrogen compounds captured and accumulated in the absorption liquid are efficiently transferred to the anode chamber side through the anion exchange membrane. Subsequently, the inventors discovered that by adding calcium hydroxide to the anolyte discharged from the anode chamber, the sulfur compound could be easily separated and recovered as calcium sulfate, and the present invention was completed.

That is, the present invention provides (a) 2 to 1000 mmol/l of ferrous ion, and (b) 400 to 1500 mmol/l of sulfite ion or hydrogen sulfite ion or a mixture of both.
Exhaust gas is purified using an absorption liquid with a pH of 5 to 8 containing 10 to 3000 mmol/l of alkanolamine and (c) alkanolamine present in the absorption liquid by a diaphragm electrolysis method using an anion exchange membrane as a diaphragm. Transfer the anionic sulfur compounds and/or nitrogen compounds from the cathode chamber to the anode chamber, and then add calcium hydroxide to the anolyte discharged from the vaginal anode chamber to generate and separate calcium sulfate. The present invention provides an exhaust gas treatment method characterized by recovery.

The present invention includes the steps of circulating the absorption liquid to the exhaust gas purification zone of the exhaust gas treatment device and the cathode section of the diaphragm electrolytic cell, and the step of treating the anolyte of the electrolytic cell.

In the absorption liquid circulation process, the absorption liquid absorbs harmful substances such as nitrogen oxides and sulfur oxides in the exhaust gas in the exhaust gas purification zone, is then guided to the cathode part of the diaphragm electrolytic cell, and is regenerated by electrolytic reduction. The exhaust gas is returned to the purification zone and subjected to exhaust gas purification treatment.

Note that the contact between the exhaust gas and the absorption liquid is not particularly limited, and may be carried out, for example, in the exhaust gas purification zone by a countercurrent or cocurrent method.

The sulfite ion or hydrogen sulfite ion used here, or a mixture of both (precipitate, solution
It is important to add 400 to 1500 mmol/l of ions (including those present as O, i ions) to the absorption liquid.

If the amount added is within this range, the absorption liquid will be in the form of a solution or suspension, but if the amount added is a trace amount (30 mm IJ mol/
(1 or less), the absorption liquid becomes slurry-like, making it difficult to process by diaphragm electrolysis.

Moreover, the pH of the absorption liquid is set to 5 to 8.

If the pH is less than 5, the absorption capacity will decrease, and if the pH is more than 8, the sulfite ions will be easily oxidized to sulfate ions, which is not appropriate, and the solubility of iron ions will also decrease.

As a ferrous ion source, ferrous sulfate, ferrous chloride, ferrous nitrate, etc. are used, and as a sulfite ion or hydrogen sulfite ion source, in addition to sulfur dioxide gas, sodium sulfite, rum, sodium hydrogen sulfite, and sulfite are used. Metal sulfites or metal hydrogen sulfites such as calcium, calcium hydrogen sulfite, ammonium sulfite, ammonium hydrogen sulfite, etc. can be used.

Examples of alkanolamines used as other components of the absorption liquid include monoalkanolamines such as monoethanolamine and monopropanolamine; dialkanolamines such as jetanolamine and dibrobanolamine; triethanolamine and triethanolamine; Examples include trialkanolamines such as propatoolamine.

A diaphragm electrolytic cell with an anion exchange membrane as a diaphragm is used for the electrolytic reduction tank used in the absorption liquid regeneration process. Nitrogen oxides, sulfur oxides, etc. that are reduced to iron ions and transferred from exhaust gas to absorption, or that are generated in the absorption liquid after transfer, pass through the anion exchange membrane as anionic substances and transfer to the anode side. It is played by

In addition, commercially available anion exchange membranes, such as those whose base material is styrene-butadiene polymer, styrene-divinylbenzene polymer, styrene-vinylpyridine-divinylbenzene polymer, polyethylene-styrene graft polymer, etc., can be used. can.

Although it is expected that sulfur compounds migrate in this process, the fact that nitrogen-containing substances migrate toward the anode side at the same time is truly unexpected, and forms a characteristic part of the present invention. It is something.

In addition, when a cation exchange membrane or bisque fired material is used as the diaphragm material, it is also possible to move and remove a part of the anionic substance, but in this case, the movement of the anionic substance to be removed This is not preferable because a sufficient separation effect cannot be obtained due to insufficient separation or movement of iron ions.

On the other hand, dilute sulfuric acid, Na 2 S
Although an electrolyte aqueous solution such as 04, K2SO4, etc. is used, it is most preferable to use dilute sulfuric acid in order to facilitate the recovery of transferred substances.

In this process, the sulfur compounds that migrate from the cathode side to the anode side are anionic sulfur oxides, specifically sulfite ions and hydrogen sulfite ions generated from sulfur dioxide gas in the exhaust gas or existing from the beginning. Or sulfate ions that have undergone partial oxidation.

In addition, nitrogen compounds are mainly anionic imide and amide compounds, and nitrogen oxides in the exhaust gas react with sulfite ions or hydrogen sulfite ions in the absorption liquid, resulting in the formation of hydroxyamine disulfonic acid, nitrilotrisulfonic acid, etc. It is thought that it was produced via an intermediate.

Next, in the process of treating the electrolytic cell anolyte, the anolyte containing anionic nitrogen compounds and sulfur compounds that have migrated to the electrolytic cell anode side is guided out of the electrolytic system, and calcium hydroxide is added thereto. Let's do it.

As mentioned above, among the sulfur compounds and nitrogen compounds that have been transferred to the anolyte and separated from the absorption liquid, most of the sulfur compounds are quickly anodic oxidized and converted into sulfate ions.

Therefore, when calcium hydroxide is added to the anolyte in such a state, extremely high purity calcium sulfate can be produced and recovered in just one step.

In this regard, considering that the conventional method requires an additional oxidation step because phlegm and calcium sulfite are produced, it will be understood that the method of the present invention is extremely advantageous and highly practical.

On the other hand, anionic compounds containing nitrogen remain in the solution, but they can be converted to sulfamic acid by adjusting the pH of the anolyte to 2 or less, and can be directly hydrolyzed or oxidized. It is possible to recover or make it harmless.

As described above, according to the present invention, it is possible to efficiently regenerate the absorption liquid used for exhaust gas purification treatment, and to recover high-purity calcium sulfate by a simple means.
Furthermore, nitrogen compounds can also be easily recovered and rendered harmless.

Next, the present invention will be explained in detail with reference to examples.

Example l FeSO4・7H20300 mmol/L Jetanolamine 2700 mmol/l, SO3"-(SO□ gas) 800 mmol/l and water were mixed, and sulfuric acid was added to adjust the pH to 7.2. 1500g was used as an absorption liquid.

The above absorption liquid was placed in a 21-volume aeration stirring tank, and 2% concentration NO gas diluted with helium gas was passed therein at a rate of 11/min for 6 hours.

Note that the temperature of the absorption liquid was maintained at 60°C.

Next, the absorption liquid used in the treatment was subjected to electrolytic reduction in an electrolytic cell having an H-type cell made of acrylic resin.

The electrolysis conditions at this time were as follows. cathode
Liquid: Absorbing liquid 1000g Anolyte: 1N-H2SO410009 Cathode and anode: Both platinum diaphragms Membrane: Anion exchange membrane (product name: Ceremion AMV, manufactured by Asahi Glass) Cathode potential: -0,65V Temperature: 50°C Bath voltage: 5■ Bath current: IA As a result, Fe2+/(Fe2
++Fe"") was 34 mol%, whereas it was 86 mol% after electrolytic reduction.

On the other hand, of the 175 mmol of imide and amide nitrogen compounds (70% imidosulfonic acid, 30% sulfamic acid) initially present in the catholyte, 27 mmol moved to the anolyte side.

At the same time, out of 1690 mmol of SOx ions, 11
4 1.1 mol moved to the anolyte side.

Among the transferred nitrogen compounds, imidosulfonic acid was hydrolyzed and the entire amount was converted to sulfamic acid.

In addition, the reduction current efficiency to divalent iron at this time is ', 'i
o.

%Met.

500 g of anolyte containing SOx ions and sulfamic acid after electrolytic reduction (57 mmol of SOx ions, 1N-
314 mmol of calcium hydroxide (containing 250 mmol of H2SO4 and 14 mmol of sulfamic acid) were added with stirring to obtain 301 mmol of calcium sulfate precipitate.

As a result of X-ray diffraction analysis of this calcium sulfate, it was found that most of the calcium sulfate was dihydrate, and only a very small amount of calcium sulfite was present.

Furthermore, the produced calcium sulfate was white, and as a result of compositional analysis, no impurities such as heavy metals were detected.

On the other hand, 14 mmol of sulfamic acid remained in the P solution.

Example 2 FeSO4.7H20 100 mmol IL jetanolamine 2600 mmol/L sulfite ion 800 mmol/L and water were mixed, and sulfuric acid was added to the mixture to adjust the pH to 7.
The liquid adjusted to 0 was used as the absorption liquid.

Simultaneous desulfurization and denitrification experiments were conducted using this absorption liquid in a 30 NM3/H midget plant for exhaust gas treatment.

The gas composition at the inlet of the flue gas absorption tower is NO300ppI
I11SO22000ppIl11025%, CO□1
2%, and the denitrification rate and desulfurization rate at the outlet of the absorption tower were maintained at 85% and 96%, respectively.

The absorption liquid composition in steady state (pH approximately 6,8) is Fe2
+65 mmol/11Fe3+35 mmol/l.

Jetanolamine 2600 mmol/l, sulfite ion 760 mmol/13. Sulfate ion 765 mmol/
It is l.

A part of the circulating absorption liquid was introduced into the cathode chamber of the diaphragm electrolytic cell to regenerate the absorption liquid.

The electrolysis conditions at this time were as follows.

Cathode liquid: Absorption liquid Anode liquid: 1N-H2SO4 Cathode: Lead Anode: Lead peroxide membrane Membrane: Anion exchange membrane (product name: Neoceptor ACH45T, manufactured by Tokuyama Soda) Cathode potential: -0,6
5V Temperature = 55°C Bath voltage: 45cm Current density: 15mA/i The absorption liquid was circulated at a rate of 201/min.

As a result, Fe2+/(
Fe"+Fe") was 59 mol%, while it was 71 mol% at the electrolytic cell outlet.

The current efficiency at this time was 94%.

On the other hand, imide and amide nitrogen compounds (imidosulfonic acid and sulfamic acid) captured in the absorption liquid move to the anode side at a rate of 295:: IJ mol/hour,
At the same time, SOx ions also moved toward the anode at a rate of 2630 mmol/hour.

Analysis revealed that most of the imidosulfonic acid transferred was hydrolyzed to sulfamic acid.

Furthermore, among the SOx ions that moved, 803'- was quickly anodized to become So4'-.

After electrolytic reduction, 1617 mmol of calcium hydroxide was added to the anolyte containing 300 mmol of SOx ions, 34 mmol of sulfamic acid, and 1,300 mmol of IN-H2SO4 with stirring, and 157 mmol of calcium sulfate was precipitated.
0 mmol was obtained.

As a result of X-ray diffraction analysis of this calcium sulfate, it was found that the calcium sulfate was mostly calcium sulfate dihydrate, and the amount of calcium sulfite was extremely small.

Furthermore, the produced calcium sulfate was white, and as a result of compositional analysis, no impurities such as heavy metals were detected.

On the other hand, 34 mmol of sulfamic acid remained in the filtrate.

Claims (1)

[Claims] 1 (a) Ferrous ion, 2 to 1000 mmol/L (
b) 400 to 1500 mmol/l of sulfite ion or hydrogen sulfite ion or a mixture of both; and (c)
The exhaust gas is purified using an absorption liquid with a pH of 5 to 8 containing alkanolamine, 10 to 3000 mmol/l, and then the anions present in the absorption liquid are purified by diaphragm electrolysis using an anion exchange membrane as a diaphragm. By transferring sulfur oxides and/or nitrogen compounds from the cathode chamber to the anode side, and adding calcium hydroxide to the anolyte discharged from the hindlimb anode chamber, calcium sulfate is generated, separated, and recovered. Characteristic exhaust gas treatment method.