DE2324547C3 - Process for removing sulfur dioxide gas from combustion exhaust gases - Google Patents

Description

be dem ais ftuwi »" »·· ---

is turned. In this process, however, the Looked for activated carbon for recovery Be a sign, and there is only a complete Ss Desorption process available. The sulfur dioxide thus obtained must also be put into a uSdH the connection is transferred, and be, difficult work is required for this process.

^a0g 1fS ^fO _a ^r u ^d c ^e h ^iC a wet desulphurization process known in which an aqueous solution of Ammontumsu fit is used as an absorbent. Be. In this vXrni the sulfur dioxide reacts with ammonia

_a5 SuIfH in the water medium, whereby ammonium-Ϊ is formed. In this process, ammonium sulphate is obtained by neutralizing the ammonium bisulphite with free ammonia; it is then converted to ammonium sulphite

dn S of the ammonium sulphite solution are oxidized, using a known method. The rest of the solution is saved as a Recydized absorbent (see French patent specification

Recently, the same applicant, from from which the present application originates, proposed a wet desulfurization process in which Be coke oven gas as a source for the free ammonia. special circulation conditions are used

HMCl I UlIU UlC iTiuuvii »u ₆ v ... _ow

circulating solvent system of step (c) recirculates.

2. The method according to claim 1, characterized in that in step (g) a part of the same time circulating solution of step (c) is added to the mother liquor before neutralization, and that the supernatant clear solution in the co-circulating solvent system is recycled to stage (c).

3. The method according to claim 2, characterized in that the precipitates from impurities, which have settled are filtered off and that the mother liquor removed from the impurities was stripped to the washing water of step (f) is added.

4. The method according to claim 1, characterized in that 40 to 80 μ wide and 150 to 300 μ long crystals of calcium sulfate dihydrate are added as seed crystals to step (d).

The invention relates to a method for removing harmful sulfur dioxide from exhaust gases with an oil heated kiln or an ore sintering device U remove, in particular at the same time as

the Ammoniuiiisuaai> »« ...., ..

gained from it. On the other hand, since ammonium sulfate, which is used as a fertilizer, is made from synthetic

Ammonia is produced in enormous quantities at a very low cost, possesses the aforementioned Reclaimed ammonium sulfate has little economic value and only brings new difficulties with yourself, because even if you throw it away, you will

thereby polluting the environment and harming the public.

The present invention is based on the object of the Belgian patent 7 84 594 to improve the procedure described by attending

Place of the uneconomical ammonium sulfate as

By-product calcium sulfate dihydrate with large

Crystal size creates a great value at

owns industrial applications.

The invention is a method to

Remove sulfur dioxide gas from a combustion exhaust gas by performing the following steps:

a) treated the exhaust gas containing sulfur dioxide gas with a circulating aqueous solution which Ammonium sulfite, ammonium bisulfite and ammonium sulfate mixed together, the pH of the solution is in the range from 5.8 to 6.5,

b) ammonia gas circulating with another aqueous solution that mixes ammonium sulfite, ammonium bisulfite and ammonium sulfate together contains, treated, the pH of the solution being in the range from 6.3 to 7.0,

c) continuously gives a portion of the circulating solution of stage a) to that of stage b) and also gives the same amount of the circulating solution of stage b) to that of stage a), characterized by the following further stages: that one

d) continuously withdraws part of the simultaneously circulating solvent mixture of stage c) and reacts it with milk of lime in the presence of large seed crystals of calcium sulfate dihydrate at normal pressure at a temperature of 90 to 100 ^° C., the pH being 11 to 12 and the resulting free ammonia gas is introduced in stage b),

e) blowing air into the slurry of step d), the pH of which by adding dilute Sulfuric acid is kept at 2 to 4,

f) the resulting slurry from step e) is filtered and then washing the calcium sulfate dihydrate cake with water, and

g) the mother liquor, which was separated off in step f), is neutralized with ammoniacal water and recycling the mother liquor into the simultaneously circulating solvent system of stage c).

According to the German Offenlegungsschrift 22 27 952, sulfur dioxide contained in the exhaust gas of a combustion furnace is combined with ammonia gas, which originates from a coke oven gas, in order to finally obtain it as ammonium sulfate. In contrast to this, _{according to the invention, the SO 2} gas is combined with CaO, which comes from milk of lime, in order to finally obtain it in the form of calcium sulfate dihydrate crystals. Ammonia is recirculated as a reaction medium in the circulating system, the amount of ammonia added to the circulating system to compensate for the composition of the circulating liquids being very small. On the other hand, according to German Offenlegungsschrift 22 27 952, ammonia gas is never used as a circulating medium, but only as a raw material. Furthermore, according to the invention, no ammonium thiosulphate is formed in the circulating liquids, while this is always the case according to the above-mentioned German patent application since coke oven gas contains H ₂ S-GaS. Furthermore, according to the invention, calcium sulfate dihydrate of large crystal size is obtained as a by-product, which is industrially very valuable, while the ammonium sulfate obtained according to the above-mentioned German Offenlegungsschrift is an almost worthless by-product.

According to the reference "Staub-Reinhaltung der Luft 2" (1968), p. 104, calcium sulfate and calcium sulfite are obtained by adding milk of lime to the circulating solution, ammonia being recovered and recirculated, and the recovered ammonia being mixed with the exhaust gas. According to the invention, on the other hand, the recovered ammonia is added to the circulating solution after the SO ₂ contained in the exhaust gas has been absorbed. Furthermore, this literature reference does not contain any teaching relating to the oxidation of calcium sulfite in order to obtain calcium sulfate alone.

In the following, a specific embodiment of the present invention will be explained in detail described. The drawing shows a flow diagram of the method according to the invention. 1 means the combustion exhaust gases from a boiler or the exhaust gases from an ore sintering device that is in the SOj scrubbing tower 2 can be introduced on the ground. An aqueous solution 3 consisting mainly of ammonium sulfite is introduced into the upper part of scrubbing tower 2 after it has been temporarily placed in a storage tank 4 had kept. Upon contact with the ammonium sulfite solution 3 in the scrubber tower 2, the sulfur dioxide gas in the exhaust gas 1 reacts with the ammonium sulfite of the solution 3 and is almost completely absorbed in it. The exhaust gas stripped from the sulfur dioxide is released into the The atmosphere is discharged through a chimney 5.

Part of the solution in the storage tank 4 is transferred to another storage facility via the lines 6 and 7.

ao tank 9, which is attached _{below the NH 3 -Skrubberturms 8.} On the other hand, a part of the solution in the storage tank 9 is returned to the storage tank 4 via the pipe 11 and thereby becomes a communal or connected circulating one

»5 solvent system formed.

Part of the solution in the storage tank 9 is circulated through a line 10 into the head part of the NH ₃ scrubber tower 8 and flows downwards there. Ammonia gas 14 discharged from the reaction vessel 13, which will be described later, enters the NH ₃ scrubber tower 8 at the lower part. All of the ammonia gas 14 essentially reacts with the ammonium _{bisulfite of the above-mentioned circulating solution in the NH 3} scrub tower 8 and is absorbed therein in the form of ammonium sulfite. Waste gas resulting from the absorption of the ammonia gas is discharged into the atmosphere through another chimney 15.
In the above-described circulation systems (equal to circulating system or circulating systems), the pH of the circulating solution 3 is regulated so that it falls within the range of 5.8 to 6.5, and the pH of the solution passing through the pipe 10 flows is regulated to be in the range of 6.3 to 7.0. When

Should it be necessary for this control, a suitable amount of fresh ammonia gas 16 is added to the NH ₃ scrub tower 8. Both circulating solutions contain ammonium sulfite, ammonium bisulfite, and ammonium sulfate.

Since the sulfur dioxide, which is contained in the exhaust gas, increasingly accumulates in the circulating solutions, part of the solution in the storage tank 4 is removed via the lines 6 and 12 after a certain period of time and passed into a reaction vessel 13, which is made with milk of lime a storage tank 17 is supplied via a line 13. Ir the vessel 13 is a reaction at normal pressure and at a temperature of 90 to 100 ⁰ C., Tatt, wherein the calcium sulfite described later unc

Calcium sulfate are precipitated and free ammonia gas is formed. In this case, e required that the pH of the slurry in the reaction vessel 13 is adjusted to 11-12 and that large seed crystals of calcium sulfate

Dihydrate are filled into the reaction vessel 13 before a reaction takes place. The ammonia gas that is formed is directed into the NH ₃ scrubber tower as previously mentioned. To the loss of a

To balance ammonium ions in all circulation systems, it is, as already mentioned above, possible fresh ammonia gas 16 into the NHp scrubber tower 8 to direct. Otherwise, an aqueous solution of ammonium sulfate (19) in the reaction vessel 13 via a Line 20 are introduced.

A slurry formed in the reaction is added to the pH regulating tank 22 via a Line 21 passed where the pH of the slurry by adding dilute sulfuric acid from a concentration of about 10% is regulated to be about 4. After that, the Slurry passed into an oxidation tank 24 via line 23. Will air in the slurry blown, the calcium sulfite and calcium bisulfite contained therein are converted into calcium sulfate. Large calcium sulfate dihydrate seed crystals already present in the slurry make it easier the further growth of the crystals. At this point the slurry has a pH of 2.0 to 3.5. If one wishes to obtain large crystals of calcium sulphate dihydrate, it is advisable to use essentially approximately half the amount of the slurry is added to the pH regulating tank 22 via a line 25 guide to extend the growing season. The remainder of the slurry is passed into a filter device 27 passed through a line 26, where the precipitated crystals of calcium sulfate dihydrate from the mother liquor be separated. The filtered precipitate of calcium sulfate dihydrate is washed with fresh water washed. This is not shown in the drawing. The waste water that comes from washing is used to make milk of lime or a solution of ammonium sulphate, as mentioned before.

The mother liquor, which is separated by the filter device 27, is passed through a line 28 into a Neutralization tank 29 passed where the sulfuric acid content of the mother liquor with ammonia water is neutralized, which is added so that the pH of the mother liquor is 7 to 8. It is essentially, the neutralized mother liquor directly into the stream of solvent mixture flowing through the Line 7 flows back. However, as will be described later, the mother liquor still contains 0.04 to 0.05 percent by weight calcium ions dissolved in it and the solution of the circulating together Systems contains small amounts of iron and aluminum compounds as impurities. In order to prevent the circulation systems from becoming clogged with scale deposits, the mother liquor will passed through a line 30 into a precipitate separator 31, in which one also part of the co-circulating flow via line 32 and also an appropriate amount of Passing ammonia water to regulate the pH of the resulting mixture to 7.0. Because about this When deposits of calcium sulfite, iron hydroxide and aluminum hydroxide are formed, it is advisable to put the slurry collected at the bottom of the precipitate separator 31 in the filter device 27 to pass through a line 33 to separate the precipitates from the solution and a supernatant clear liquid obtained in the upper part of the separator 31 via the line 34 to lead into line 7. The mother liquor obtained from the filtered cake of the aforementioned impurities should preferably be added to the washing water that is used, to filter the by-product, calcium sulfate dihydrate.

As the circulating water increasingly _{evaporates in the SO 2} scrub tower 2 and the NH ₃ scrub tower 8, the amount of circulating water can be gradually decreased in all circulation systems. However, the water loss is increased by the addition of ammonia and dilute sulfuric acid at various points to the circulation systems

ίο as well as by checking the storage tanks 17 and 19 the amount of water added to the milk of lime and ammonium sulfate can be balanced the entire device according to the invention can be operated continuously, the amount of liquid in the circulation systems are well balanced.

The following are the important features and Apparatus parts to do with the circulation systems in the present invention in more detail

ao explained.

I. pH values of the circulation solutions in the
Scrubber towers (= washing towers)

As mentioned earlier, the solution _{flowing through the SO 2} scrub tower 2 contains ammonium sulfite, ammonium bisulfite and ammonium sulfate, and it is necessary to regulate the pH of the solution so that it has a value that is in the range of 5, 8th

up to 6.5. The higher the pH, the more effective the desulfurization. In this case, however, the partial vapor pressure of the ammonia becomes meaningful and light ammonium sulfite vapors are produced. Depending on the capacity of the SO ₂ scrubber tower 2, the desulphurisation efficiency is generally more than 90% when the pH of the solution in the washing tower 2 is 5.8 and substantially 100% when the pH is 6 .5 is.

The solution that runs through the NH ₃ scrub tower 8 and which has already absorbed ammonia gas, of course, has a higher pH value than the solution that runs through the SO ₂ scrub tower 2, the higher pH value being chosen so that he in

Range from 6.3 to 7.0.

II. Growth of large crystals on
Calcium sulfate dihydrate

The growth of large crystals of calcium sulfate dihydrate, which is a by-product, has been by appropriately controlling the operating conditions in the reaction vessel 13, the pH regulating tank 22 and the oxidation tank 24 possible.

If the sulfur dioxide gas is first absorbed in ammoniacal water, the ammonium sulfite becomes formed according to the following equation (1):

2NH ₄ OH + SO ₂ = (NH ₄ ) ₂ SO ₃ + H _i O (1)

Part of the ammonium sulfite formed reacts! with sulphurous acid and is converted into ammonium bisulfil converted according to the following equation (2):

= 2 NH ₄ HSO ₃ (2)

At this point, another part of the ammonium sulfite in the solution will react with the oxygen gas, which is contained in the exhaust gases, and thereby becomes

Ammonium sulfate formed. Correspondingly, there are ammonium sulfite, ammonium bisulfite and ammonium sulfate at the same time in the circulating solutions.

If milk of lime is added to the withdrawn part of the circulating solution, calcium sulfite becomes and calcium sulfate precipitated and ammonia gas is according to equations (3), (4) and (5), the consequences, released:

10

(NH ₄ J ₂ SO ₃ H-Ca (OH), - CaSO ₃ -1/2 H ₂ O + 2 NH ₃ T -fl ¹ / «H ₂ O (3)

NH ₄ HSO., - I Ca (OH) ₂ ->
CaSO ₃ ■ I / 2 H ₂ O + NH ₃ T + 1V ₂ H ₂ O (4)

15th

(NH ₄ ) ₂ SO, + Ca (OH) ₂ - *
CaSO ₁₁ -2 H ₂ OI 2 NH ₃ T

(5)

As previously mentioned, the ammonia gas formed is _recovered in the NH ₃ scrubber tower 8.

The calcium sulphite precipitate found in the slurry obtained according to the aforementioned receives double conversion, has to be converted into useful calcium sulfate. In which In known methods, the sulfur dioxide gas was absorbed in the slurry to make insoluble calcium sulfite converted into soluble calcium bisulfite, the pH of the slurry being reduced to about 4 was decreased.

Air was then blown into the slurry to oxidize the calcium bisulfite to insoluble calcium sulfate dihydrate. The dihydrate thus produced contained fine crystals that are 5 to 10 μ wide and 20 to 40 μ long. Difficulties arise when the crystals are filtered off from the mother liquor. Even when filtered off, such fine crystals cannot be used as plaster of paris material in the manufacture of plaster pieces or blocks. It is difficult to produce with high concen- Schwefeldioxydgas ₄ "concentration, except in sulfuric acid production plants. In the known method it is therefore impossible to use sulfur dioxide gas with such a low concentration as is obtained in the exhaust gases from boilers heated with oil.

In order to solve the above problems, there has been proposed a method of producing calcium sulfate hemihydrate from the ammonium sulfate contained in the slurry according to the following equation (6) by performing the double conversion at a pressure of 2.5-4 , 0 kg / cm ² and at a temperature higher than 125 ° C:

(NH ₄ ) ₂ SO ₄ + Ca (OH) ₂ -

CaSO ₄ 1/2 H ₂ O + 2 NH ₃ + 1V ₂ H ₂ O (6)

CaSO ₄ · 1/2 H ₂
CaSO ₄ · 2 H ₂ O

H ₂ O

(7)

However, the double reaction carried out at the above-mentioned high pressure requires

a complicated specialized device and difficult to implement.

According to the claimed process, the double conversion is carried out at normal pressure at special Conditions carried out. In doing so, the solvent mixture comes from the connected circulation system is withdrawn, milk of lime added so that the pH of the slurry, which at the double reaction is formed, in the range from II to 12 is. With this regulation, the Amount of CaO added 1.01 to 1.05 times the theoretically equivalent amount required is to react with the ammonium salts contained in the solvent mixture. Γη In this case, the reaction temperature can be kept in the range from 90 to 100 ° C. It is also required a certain amount of large crystals of calcium sulfate dihydrate in the solution, which is withdrawn will give before double decomposition. These conditions enable in the slurry which occurs in the double conversion, a pronounced growth of large crystals Calcium sulfate dihydrate that are 30 to 60 μ wide and 100 to 250 μ long. These crystals are also used as seed crystals to encourage further growth of large crystals of calcium sulfate dihydrate to be activated in the subsequent oxidation stage.

III. Oxidation of the precipitate
Calcium sulfite in the slurry

The slurry resulting from the double conversion contains large crystals of calcium sulfate dihydrate, small crystals of calcium sulfate hemihydrate and a small amount of calcium hydroxide. A suitable amount of dilute sulfuric acid at a concentration of approximately 10% is added to the slurry to maintain the pH at about 4. Calcium sulfate dihydrate crystals fall in the process according to the following equations:

CaSO ₃ · 1/2 H ₂ O + 1V ₂ H ₂ O + H ₂ SO ₄
CaSO ₄ -2 H ₂ O + H ₂ SO ₃

CaSO ₃ - 1/2 H ₂ O + H ₂ SO ₃ Ca (HSO ₃ ) ₂ + V ₂ H ₂ O

Ca (OH) ₂ + H ₂ SO ₄ -CaSO ₄

2H ₂ O

(8th)

(9)
(10)

When air is blown into the slurry, the calcium bisulfite dissolved therein becomes calcium sulfate dihydrate according to the following equation (11) transferred:

55 Ca (HSO ₃ ) ₂ +2 H ₂ O + 1/2 O ₂
CaSO ₄ · 2 H ₂ O + HjSO ₃

(H)

The calcium sulfate hemihydrate has a greater solubility in water than the dihydrate, so that if the hemihydrate further into the dihydrate according to the following equation (7) during the oxidation stage, which follows the double decomposition is hydrated, the growth of the crystals of the dihydrate is activated:

The sulphurous acid obtained as a by-product is consumed in dissolving calcium sulfite according to the above-described equation (9). The formation of this sulphurous acid lowers the pH of the slurry to 2.0 to 3.5.

The crystals of calcium sulfate dihydrate, which nact the equations (8), (10) and (11) are precipitated, are extremely fine and therefore easily soluble, so that the seed crystals consist of calcium sulfate dihydrate which are already present in the slurry, gradually with the dissolution of the fine crystals from the precipitated dihydrate become larger. For a full

4ο

Growth of the crystals it is desirable to approximate a part of the circulating solution with a,

half of the slurry, which is taken from the Oxyda- amount of 7.1 l / h and converted into a reaction *

tion tank 24 is taken into the pH regulating vessel 13, in which also milk of lime, which

tank 22 to contain the growth time to 9.92 percent by weight CaO, with a

extend. ₅ speed of 1.61 l / h is introduced. In the

The reaction vessel is further made from an aqueous solution

IV. Treatment of the mother liquor produced by the calcium ammonium sulphate with a concentration of 20

suIfat dihydrate crystals were separated · weight percent at a rate of 0.3 l / h

bestl7T % 1r ^P V, 7E, 7 i ^bis , ^{3 '51} ^ LZ% L iXlUlSSSiS

mi de, RlterT h, ^Calci "™ ^su ' ^fa -D'hydratkr.stallen Calcium sulfate dihydrate, which was separated off as a by-product with the filter device 27, is obtained in the present invention in the

■ · sas ?; '

genes can form. It is therefore preferred, as previously free, n _ae a ™ J T ¹ V ^g % ■ j

mentions that the calcium ions are removed by * o wf d η _e "en NH Skn h ^? ' V ° iT '

they are used together with other impurities, such as a solution dtdiSh ΐ τ "™ ^{8 8} ^ L,
Iron and aluminum compounds contained in the Z rku- 330 / hf5 absSbiel, w "ΪΤ" VT 'r ^ r Γ lation solutions, precipitates. Becomes part of the de! Am «ΐ P" ^A Jf ^or P ' ^Ionsfahl g-

of the stream L solvent mixture to the MuUe 'o ägt 97 ^ / JnS Te ^ nH W ", f". ^S .k ™ «» bertunn 8 lye added, the pH value of which is still low, .5 ^ gfmTuTmii ^ riS '^ ^m
then soluble calcium sulfate Dihydra readily forms to the ÄukchS ⁸ h ", etc. a. ..

and only about 50% of the calcium ions will se ^ Ä ZT "ΐκ Τ ^

regained. However, it has been found that there is a? Concentration t £ F ™ , ^{sulfuric acid with} the maintenance of the pH value of the mother liquor at 7.0 Sen " fvonTs VnH uZ! T ^ WT ' by adding ammoniacal water and 30 munf to" ™ " T ^ ^H ^ ^{eTt the} pubic part of the flow of solvent mixtures ZSi ^ AS '* -? "^Ve; ^mmd f ^rn ; ^D ! ^{e Aufs} ä'äm" enables about 90% of the calcium ions, the Γη "5T ^ S Z lZ ! T" L ^> te , ner amount are contained in the mother liquor, in the form of little _R schlämmunT ti "u λ T ^W ° ^be ' ^{da $} '" ^d " ^A , ^on " soluble calcium sulfite are precipitated Κοηη _β Γ _ω η5 DihvH 1! Pu * ™ - ^ ^{lciumsulflt in} calcium sulphate that the precipitates of iron hydroxide ZZ Alu 35 Ä ^ ^ - ^{mT ± Un} ß ^{about the half of the} miniumhydroxyd are mixed with it. Since Sl, the "tankfhl t * ^{HCH] a} _A ^MMUN Z ^<of the oxidation-precipitates fine crystals 2-20 μ s hai en S ΪΡ f ^ n ^{W 'RD', is returned in the} PH-Reguliersind several hours required until the SediSSSSn 'Srd mt efner Rt? ^ "^ 'ri ^Aufschl t ^mUng terminated, st. The application of a suitable coagu- dn« i Nied ^ hS? ¹ , "" ⁸ ^ "V ^'^"

lierungsmittels for the precipitation allows IE 4o * "™ JNiederschlag from Calc.umsulfat dihydrate in but that the sedimentation in a few minutes IO h ^ ff T ^{4 '5 kg (Tr} ockengewichtsb _a sis) / h and finished 10 minutes is l / h mother liquor is obtained.

Sulphate dihydrate is made up of large crystals that

V. Advantages of the method according to the invention irh »hti? MS *? ^And ' ⁵ ° ^b ' ^{S 30G μ long sin (L} ° ' ^e

, _Δ -v-Ai-i, - «mother liquor obtained has a pH of 2.3.

1. Ammonia is used as a circulating absorption ⁴⁵ pe various ions used entmittel in the mother liquor. The required fresh addition ^h ^ are ten, are in the following Konzentravon ammonia is low. before:

2. The sulfur dioxide gas contained in the exhaust gas- NH ₊

th is in the form of calcium sulfate dihydrate r Λ percent by weight

^Obtained from a large crystal size and can be expressed as ⁵ ° SO - ° ' ^{044 percent by weight}

Raw material for Portland cement or gypsum board sn - ^ ^{009 weight} P ^rcent

be used. ⁴ ~ 0.088 weight percent

3. Waste liquid does not fall from any part of the system. The calcium sulfate dihydrate obtained is with

₅₅ vTκΓ ^{Γ washed and} dried and forms the

The following example illustrates the Erfindune In ntt ^ xi ^{kL The Mutterl} eye is with a geeigdem example in the flow diagram (taken Zeichnunp ^ c -Z ß ^e niakalischem ^{to amm} ° water neutrali reference. Guild) si _ert to the pH Set the value from 7 to 8, and

_1Λ £ ^{Μ AbgaS} * from ^{an er2 sintening} device, which contains dmStf ^^ ¹ ^ ⁰ ^ ¹⁸ * ⁸ ^ ¹ "" ¹¹⁰¹ " ^{31 gcbcn} · ^" 1000 ppm sulfur dioxide, becomes in the SO 60 "" f! ^ ^Wrd a simultaneously circulating Strom Skrubberturm 2, into which with a sight windie ' r ^os ^! ^8SIittelii schung with an amount of 2 l / h speed of 1 m ^{3 / h a circulation solution} the 7 36 Ge rf · Nledcrschl agseparator 31 via a line 32, weight percent ammonram sulfite, 7.31 weight percent n! T * ^Zlr kulationsleitung 7 branches off, carried ammonium bisulphite and 16.20 percent by weight Am ΛΛ ^{xpaiaXor 31} is also carried out with a suitable moniumsulphate. The Tenroeratu- 6s aT " ⁸ " ^? / ^Ajnmoniak al'schem water and the pH of the solution are at 50 ° C or h! °? " ^{Of the} S ⁶ Sa ¹ DtCn content of the separator 31

held at 6.2. As a result, the SO, content mi / ™ ^lten · ^Nied erschläge of calcium sulfite, the exhaust gas to 20 ppm reduced "senüydroxyd and Alummiumhydroxyd that in the

separator 31 are available in small quantities,

settle after the addition of a coagulant off the ground. The resulting slurry is from the bottom of the separator 31 in an amount withdrawn from 1 l / h and to the washing water that is used in the filter device 27 via a

Line 33 given. The supernatant clear liquid that is in the top of the precipitate separator 31 is present, is returned in an amount of 10 l / h into the line 7, through which the flow from solvent mix.: ments circulated at the same time.

1 sheet of drawings

Claims (1)

Patent claims: 1. A method to remove sulfur dioxide gas from a combustion exhaust gas by performing the following steps:
i) treating the exhaust gas containing sulfur dioxide gas with a circulating aqueous solution containing ammonium sulfite, ammonium bisulfite and ammonium sulfate mixed together, the pH of the solution being in the range from 5.8 to 6.5, b) ammonia gas with another circulating aqueous solution, the ammonium sulfite, ammonium bisulfite and ammonium sulfate mixed together, treated, the pH of the solution in the range of 6.2 to 7.0 is c) continuously a part of the circulating solution of step (a) into that of step (b) and also the same amount of the circulating solution of step (b) in that of step (a) there, characterized by the following further stages: that one d) continuously withdrawing part of the simultaneously circulating solvent mixture from stage (c) and reacting it with milk of lime in the presence of large seed crystals of calcium sulfate dihydrate at normal pressure at a temperature of 90 to 100 ^° C., the pH being 11 to 12 and the resulting free ammonia gas is introduced into stage (b), e) blowing air into the slurry of step (d), the pH of which is added by adding is kept at 2 to 4 by dilute sulfuric acid, f) the resulting slurry of step (e) is filtered and then the calcium sulfate dihydrate cake washes with water, and g) the mother liquor which is separated off in step (f) By-product calcium sulfate dihydrate can win large crystal size. important problem, for a long time it has been extremely _{detrimental to em}