JPS5922572B2 - Method for removing sulfur oxides and nitrogen oxides from exhaust gas - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention deals with the treatment of sulfur oxides (hereinafter abbreviated as SOx) and nitrogen oxides (hereinafter abbreviated as NOx) contained in general combustion gas discharged from boilers, combustion furnaces, etc. ), it relates to a method that utilizes the properties of red mud, which is an extraction residue discharged from the alumina manufacturing process, in particular.

Conventionally, to produce alumina, which is a raw material for aluminum electrolytic refining, bauxite is treated with caustic soda, the alumina content is dissolved and separated as sodium aluminate, this is hydrolyzed to obtain aluminum hydroxide, and after separation from the medium, water is The Bayer process, in which alumina is obtained by roasting aluminum oxide in a kiln, is commonly used.

This alkaline extraction step produces precipitates (Fe.

kl, S i , Na 2 , and Ca) is generally called red mud, and various methods for effectively utilizing red mud have been proposed.

The present invention has been made in consideration of this situation, and aims to apply it to the purification of exhaust gas by effectively utilizing the alkalinity of red mud.

In other words, the present inventors focused on the alkaline content in this red mud, and used the alkaline content to collect SOx in the exhaust gas.
As a result of intensive research aimed at reducing and removing NOx at the same time, we found that red mud has sufficient properties. The present invention was developed based on the knowledge that the characteristics can be exhibited more effectively.

It is well known that in order to treat NOx at a practical rate, it is necessary to oxidize the NO in NOx, and methods include oxidizing with ozone, oxygen, or chlorine dioxide;
There are methods of oxidizing with an aqueous solution or an acidic aqueous solution of sodium chlorate, sodium chlorite, or potassium permanganate, and any of these methods are known oxidizing agents and can be oxidized.

However, both methods have problems, such as the oxidizing agent being expensive and the oxidation product containing a large amount of nitrate nitrogen, making post-treatment difficult.

As a method to solve these problems, we first introduced a method for oxidizing NO in exhaust gas (Japanese Patent Application Laid-Open No. 51-20772),
A method for removing sulfur oxides and nitrogen oxides from exhaust gas was proposed (Japanese Unexamined Patent Publication No. 51-95977).

Among these, the NOx oxidation method shown in the latter,
That is, NOx in the exhaust gas is oxidized in advance using an aqueous sulfuric acid solution containing sodium or calcium chlorate, and then brought into contact with the red mud slurry liquid described above to produce SOx.

This made it possible to absorb and remove NOx at the same time.

In this case, for the oxidation of NOx in the exhaust gas in the method of the present invention, the above-mentioned known method of bringing the NOx into contact with an aqueous sulfuric acid solution containing chlorate of sodium or calcium can be effectively used as is.

In general, red mud contains alkaline components such as Na and Ca as described above, so the slurry liquid thereof exhibits alkalinity and is known to be a good SOx scavenger.

However, when desulfurizing at a high rate, the elution of alkaline components in the red mud dominates the entire rate, and even in the case of low concentration oxygen-containing gas such as general flue gas, the elution of alkalis during desulfurization is a factor of 50. It is known that the desulfurization efficiency is poor (US Patent No. 3,298,781), and an improvement method has been proposed (Japanese Patent Publication No. 51-4087).
No. 0) This method requires oxygen or air blowing, and neither method can remove nitrogen.

Further, in the case of the method disclosed in the US patent, the concentration of the reducing substance in the discharged slurry is several parts by weight, but it is necessary to oxidize it before discharge.

In this case, the amount of reducing substances is considerably reduced due to the effects of metal ions in the red mud, but it is still high, and as mentioned above, the elution rate of alkaline components is low and the desulfurization efficiency is poor at less than 80%. be.

However, as in the method of the present invention, NOx is oxidized using an aqueous sulfuric acid solution containing sodium or calcium chlorate, and then brought into contact with red mud to oxidize chlorine-based gas, No2, etc. in the treated gas. The coexistence of aqueous gases acts as an accelerator for alkali elution, which greatly improves desulfurization efficiency.

Furthermore, NOx is also efficiently removed. In other words, the desulfurization rate is 90 coefficients or more, and the alkaline content elution rate is also 80~
It is high, over 90%, and the amount of red mud used for this purpose is reduced by almost half.

Furthermore, the amount of reducing substances in the discharged slurry liquid obtained due to the influence of red mud was reduced to 0.5 weight factor, 115 to 1/10, due to oxygen oxidation after removing NOx such as NO2.

In this way, by simultaneously desulfurizing and denitrating, it was possible to almost completely extract and utilize the alkaline content in the red mud, and the burden of post-treatment of the discharged reducing substances was also reduced by a considerable amount.

The red mud used in the present invention may be discharged from any process as long as it contains alkaline content in its solids, and at the stage of exhaust gas treatment, the slurry concentration is between 0.1 and 70% by weight. Can be used in

Although it can be used at low concentrations, a large amount of liquid must be handled depending on the SO2 in the exhaust gas.

Furthermore, when the concentration becomes high, the amount of liquid decreases, but handling becomes difficult due to pressure loss.

In addition, in order to maintain the SOx trapping ability of the slurry liquid in the tower that comes into contact with the exhaust gas, it is necessary to add red mud slurry or its soaking water from the outside to maintain the pH at 4 or higher.
At the same time, the pH was adjusted to 9 to prevent the formation of nitrite nitrogen in the liquid.
It is desirable to maintain the following.

Thus, a practical simultaneous desulfurization and denitration method was established in which SO□ was collected as sulfite and acid sulfite, and NOx in previously oxidized exhaust gas was effectively removed.

The gas-liquid (slurry) contact in each group may be in any commonly used form, such as a bubble column, plate column, perforated plate column, or spray column.

The normal operating temperature is 0 to 150℃, but the temperature is 20℃ for handling the material.
~80℃ is preferable.

Next, the present invention will be further explained with reference to examples, but the present invention is not limited to these examples.

Example 1 A bubble column having a diameter of 30 M and a height of 50 wl1 was charged with 10 yd of a 50 weight acid aqueous solution, and 1.0 yd of an aqueous solution containing 10 weight weight of sodium chlorate was supplied and mixed.

The exhaust gas discharged from the boiler factory to the bottom of this tower (
N0240p, NNOx2p1 other than NO), SO
210001) IT! [1, H2O12 capacity section, 0□2
．． 5 capacity section, CO212 capacity section, remaining N2) 123t
After blowing at a rate of /Hr and oxidizing, the diameter is 50 and the height is 30.
I blew into the bubble tower at 0 Hara.

This bubble column was previously charged with 400 slurry liquid containing 30 parts by weight of red mud, and gas-liquid contact was carried out.

An analysis of the exhaust gas at the outlet revealed that it was 80,240pp.
m, NO24ppm, and NO222ppm.

Example 2 A glass tube with a diameter of 20 M and a length of 900 H was filled with 6 Raschig rings.
Weight ratio of chloric acid to IJum aqueous solution is 30:1 by weight.
10 ml of the mixed aqueous solution added at the ratio of ! /Hr.

At the same time, flue gas (NOx 200p) at 5-0℃ from the top
P11, N0180-185pp [Il, 5O
2900 ppIn, 0□2.5 capacity, CO211 capacity, remaining N2) was supplied at a rate of 200 A/Hr.

Separately, a bubble column with a diameter of 50 mm and a height of 500 mm is installed, and the amount of retained liquid is 58 mm.
9 Go, red mud slurry concentration 25 weight.

The gas was introduced while maintaining the pH at 7 or higher.

At the same time, about 40 g of 25% red mud slurry was supplied per hour and approximately the same amount was continuously extracted.

As a result, S02 in the obtained exhaust gas was 35 ppm,
NO2ppm, after reaching steady state N0230-40
It was pym.

Example 3 Diameter 45071g 1.2 inch ring 2001 filled, 3m
A packed column (first column) with a height of 350 M and a perforated plate column (second column) with six stages of perforated plates each having a diameter of 350 M, a pore size of 6 mm, and a porosity of 15% were arranged in series.

Actual gas at 200 to 250°C was humidified and cooled to 50 to 55°C and then blown into the first tower.

SSO21300pp, NOl 43ppm, NO
The gas composition was 22 ppm, 16 to 18 parts H2O, and 0□4%, and was blown at a rate of about 400 to 43 ON7113/Hr.

The first column was sprayed with an aqueous solution containing 35-38% H2SO4 and 0.3-0.6% ClO3''.
001/Hr, and NaClO3 from the outside about 14
09/Hr was added as an aqueous solution.

The second column contains 20-20% SOe containing 0.2-0.4%
The 5 weight scale red mud slurry was circulated at 300-370 t/Hr, and the 25 weight scale red mud slurry was supplied at about 44 t/Hr to maintain pH=5-7.

SO□ in the exhaust gas passing through the first and second towers is 7
Qppm, NO7ppm, NOx 53ppm
Met.

When the first tower was removed and the humidified and cooled gas was directly blown into the second tower, SO□402p1)III, N0143
1] pm, NO□2p door, it was necessary to supply about 801/hr of 25 weight red mud slurry.

Example 4 Treatment was carried out under the same conditions as in Example 3, except that ferrous sulfate was added only to the first column liquid to suppress the formation of nitric acid or nitrates, and the concentration was maintained at 1100 pp. , SO2 in the exhaust gas is 70 pl) In, N
05pI)III, NOx 22p1)Ill.

Claims (1)

[Claims] 1 After oxidizing the nitrogen monoxide in the nitrogen oxides by contacting and reacting the exhaust gas containing sulfur oxides and nitrogen oxides with a sulfuric acid aqueous solution containing sodium or calcium chlorate,
A method for removing sulfur oxides and nitrogen oxides from exhaust gas, which is characterized by contacting with a red mud slurry liquid obtained in an alkaline extraction process for producing alumina from bauxite.