DE2916975C2 - Process for removing sulfur oxides from gases - Google Patents

Description

The removal of sludges containing calcium sulfite is one of the major environmental pollution problems. that the US is facing today. Such sludges arise in numerous industrial processes, such as the desulphurisation of exhaust gases containing sulfur dioxide, which also contain other oxides of the May contain sulfur. Such sludges often contain both calcium sulfite hemihydrate also calcium sulfate dihydrate. Most of such sludges drain very slowly and some contain large amounts of water after subjecting them to standard drainage procedures. Some are thixotropic. Much land is lost as a lagoon to store such sludge, and so are the protectionists fear that the water from collected sludge with low solids content may become in Surface water and groundwater seep out and spoil them. Large amounts of money are made through the Industry and communities to build, operate and maintain sludge control systems consumed.

Some methods of combating or preventing sludge problems have been suggested, however, none are entirely satisfactory. The previous attempts at solutions include such things like lining the lagoons with concrete or plastic cladding to prevent the water from seeping out to prevent, which is costly, the landfill without treatment, which leads to long periods of time requires partial drainage of the mud and large expanses of land and often leaves the land for others Makes useless, a forced oxidation in the exhaust gas desulphurization, so that a higher ratio from calcium sulfate dihydrate to calcium sulfite hemihydrate is obtained, which is additional costly Requires the discharge of acidic sewage into the ocean, which is required by law and equipment Regulations are limited, the filtration, which is expensive, the injection of the waste into deep disused wells, what legal regulations and the non-existence of such abandoned deep holes on the most bodies are subject to the deduction of the untreated

ten acid waste in streams of what is increasingly being banned in a number of countries, the plastering of Land with untreated mud that dries slowly and requires a lot of land to stabilize the Sludge as a solid with expensive chemical fixatives, which continues to increase the total volume of the solids to be disposed of is increased and making the land permanent for storage of the same Is to be made available, as well as the modification of the ίο crystal size by adjusting the physical Treatment conditions, which are usually very costly due to equipment and process requirements to achieve the necessary physical conditions.

The object of the invention consists in an improved method for removing sulfur oxides from gases from which an improved sludge product is obtained.

The process according to the invention for removing sulfur oxides from gases, the gases being ^{contacted at 2 to 100 °} C. with an aqueous scrubbing liquid which contains calcium ions, sulfite ions or bisulfite ions and alkali metal polyphosphate, is characterized in that the alkali metal polyphosphate is Graham's salt or one of the general Formula MiP _n O (J _{n +} I) is used, the weight ratio of Ca to Graham's salt or Ca to the P _n O (3n + i) portion of M ₁ P _n Op _{n +} I) being 1: 0 , 0015 to 0.2, M is a cation, η is a positive number greater than 1, χ is the value (n + 2) ly and y is the valence of M.

The components are preferably contacted at 2 to 60 ° C., and a weight ratio of Ca to Graham's salt or Ca to the P _n Op _{n +} ij portion of 1: 0.1 to 0.04 is maintained.

A new material, which lime or limestone and Graham's salt or a water-soluble polymer of the formula above, wherein the new material may also contain water.

The method according to the invention is particularly valuable in connection with the removal of sulfur oxides in the field of exhaust gas desulfurization, as set out in detail below.
The sludges obtained as products in the process according to the invention are valuable in that they have a faster initial settling rate than the corresponding sludges obtained without the use of Graham's salt or the compounds of the formula. The sludges obtained according to the invention are also valuable as landfill or as raw material to be oxidized which is converted into plasterboards.

The new product, which is lime or limestone and Graham's salt or a compound of the formula contains, is valuable as a product for the treatment of aqueous wastewater containing sulphurous acid. It is also valuable as a base for dilution with water or with water and other lime or Limestone, to form aqueous masses for the precipitation of calcium-containing sludge from wastewater, which contain sulphurous acid, or for contact with exhaust gas, which contains sulfur oxides, to a to form calcium-containing sludge, or for contact with sodium-containing waters, which previously Exhaust gas containing sulfur oxides contacted to form a calcium-containing sludge. Such sodium-containing waters can be found in

the double alkaline process of exhaust gas desulphurization.

It should be noted that the invention can be carried out equally well if sulfate ions are combined present with sulfite ions or bisulfite ions.

The following are some improvements according to the present invention over the prior art Technique specified.

Those produced by the process of the invention calcium-containing sludges have faster initial settling rates than their counterparts Sludges, without the participation of Graham's salt or a compound of the formula I were manufactured. If still the lower end of the range or the preferred range of Graham's schemes Salt or compound of the formula I, as indicated here, is used, have those obtained calcium-containing sludges also have smaller volumes containing solids, lower viscosities, faster filtration speeds and they contact to a greater density than the corresponding calcium-containing sludge, which were prepared without Graham's salt or a compound of formula I. In some cases was observed under a magnification of 2000 times that the normal crystal morphology of the calcium sulfite hemihydrate was modified in that the crystals agglomerated into larger spheres than normal and that the normal crystal morphology of the calcium sulfate dihydrate by the present process of was modified needle-like to block-like, which can contribute to the properties given above.

As more water can be removed from the mud, the mud lagoons become smaller and fewer needed so that land is saved. The method according to the invention can be carried out at a relatively low cost because cheap chemicals can be used in small quantities. Both that Process as well as the new mass can be attached to the present neutralizations with lime or limestone of sulphurous acid adapted only with minor addition or modification of the existing equipment will. For example, Graham's salt or a compound of the formula I can be moist or dry be introduced into the moist or dry lime or limestone before the lime or limestone die Contacted sulfite or bisulfite ions. The water released from the sludge can according to usual Measures are treated and either circulated or made to streams more rapidly and recycled in larger quantities so that the risk of water pollution is reduced and thereby reducing the storage area required to deposit the sludge. The increased Density of the obtained sludge improves the economy of the material handling, since less Time and smaller equipments are necessary to get the solids of smaller volume and higher density to move. In addition, the increased density of the stored sludge makes it seep and leak less possible. Because more water can be removed from the mud before it goes into landfills is brought or is used for land gypsum plastering, such impurities, that are removed with the water, treated biologically or chemically together with other wastewater instead of being brought into the country. The present proceedings are not supported by the fly ash usually contained in the exhaust gas and also not due to small amounts of MgO, which sometimes can be used together with lime or limestone water to neutralize exhaust gas, and also not due to the presence of sodium ions, which are used in the so-called double alkali process for desulphurisation Find influenced by exhaust gases

Numerous other advantages will continue to be apparent to those skilled in the art.

The present invention is explained in detail below.

ίο It is important that the Graharn salt or the Compound of the formula I is dissolved and is present when the calcium ions with the sulfite or bisulfite ions react. The addition of Graham's salt or the compound of formula I after the formation of the calcium-containing sludge does not give a favorable result.

In the practice of the inventive process, the calcium ions with the sulfite or bisulfite ions can, whereby sulfate ion may optionally be present at from 2 to 100 ⁰ C and preferably to be contacted by 2 to 60 ° C.

The term lime as used here denotes the various chemical and physical forms quick lime, slaked lime and hydraulic lime.

Graham's salt is a well-known soluble glass of high molecular weight which, according to some Process can be produced and this also includes the process from monobasic sodium phosphate by prolonged melting and rapid cooling of the melt.

The amount of Graham's salt or the compound of the formula I to be used in the present process depends on the amount of calcium present. Usually 1 part of calcium ions to either 0.0015 to 0.2 part of Graham's salt or 0.0015 to 0.2 part of the PnO (3 "+ i) present in the formula I is suitable and 1 part of calcium ions to 0.01 to 0.04 part Graham's salt or PnOp _{n +} I) is preferred. The parts of Ca with respect to Graham's salt or P _n O (J ₁₊ η are calculated here on a weight basis, 100% content, dry. However, if the upper end of the specified range of Graham's salt or compounds of formula I is used, the initial rate of sedimentation of the sludge

45 · considerably and the volume eventually settled can be undesirable over those the corresponding calcium-containing sludge, which without Graham's salt or compound of formula 1 were made to increase. The measure against the undesired increase of the eventually settled In terms of volume, the amount of Graham's salt or the compound of the formula I is adjusted down to the preferred area. If there are large amounts of heavy metal ions such as iron, larger amounts of Graham's salt or the compound of formula I can be used to to eliminate the iron and achieve the desired result.

The compounds of the formula I include such compounds as sodium pyrophosphate, sodium triporyphosphate, potassium pyrophosphate, potassium tripolyphosphate, and some technical acids are known to contain compounds of the formula I. the Compounds of formula 1 can be used in aqueous or anhydrous form.

The method according to the invention is valuable in connection with the removal of sulfur oxides from Gases. For example, this includes burning

Exhaust gas obtained from fossil fuels may contain sulfur dioxide and may also contain sulfur trioxide. if When sulfur dioxide comes into contact with water, sulphurous acid is formed, which is converted into sulphite and Ionized bisulfite ions If sulfur trioxide is also present, it forms sulfuric acid on contact with Water that ionizes to form sulfate ions. If exhaust in direct contact with aqueous lime or aqueous limestone is wet washed, a slurry, which contains calcium sulfite hemihydrate, formed, this slurry additionally containing calcium sulfate dihydrate, albeit sulfur trioxide in the Exhaust gas was present. By adding Graham's salt or the compound of formula 1 to the Aqueous lime or limestone before coming into contact with the exhaust gas can sludge containing calcium with the the advantageous properties listed above can be produced. The method according to the invention is also suitable for use in the so-called double alkali system of exhaust gas desulphurization, with one aqueous sodium salt solution first the exhaust gas to form sodium sulfite, sodium bisulfite and optionally Sodium sulfate contacted, which then subsequently lime or limestone with the formation of contact calcium-containing sludge and release of sodium for circulation. That The present method can still be used, regardless of whether external air is entering the system is introduced or not in order to convert part of the sulfite to sulfate, this process being called forced oxidation is known. For all of these procedures, contact one time or the other time calcium ions bisulphite ions or sulphite ions and it becomes a calcium sulphite hemihydrate containing sludge formed.

The new material according to the invention comprises lime or limestone and Graham's salt or a water-soluble polymer of the formula MjP _n Op _{n +} I), which may optionally additionally contain water and in which M, x, y and π have the meanings given above. It can be made by adding the ingredients together in any order desired. In a particularly useful embodiment of this new composition, the weight ratio of calcium to Graham's salt or the P _n O (3/7 + i> component of the compound of the formula I is 1: 0.0015 to 0.2.

Illustrative examples are given below, in which all parts are by weight, unless otherwise indicated. The values contained in the following tables were obtained from curves obtained by plotting the height of the liquid-solid boundary line of the slurry, as it was formed in the respective example, as a function of time under static conditions in a graduated cylinder of 100 ml became. The term "initial settling rate" as used here denotes the difference in the height of the liquid-solid interface between time i ₀ and time Λ. divided by time ii, during the initial linear part of the curve given above. The term "finally settled volume" as used here means, from the curves given above, the volume of the solids-containing part of the sludge below the liquid-solid interface after a period of time during which the interface height remained practically unchanged for 2 hours.

example 1

A mass containing 1 part of calcium per 0.01 part of Graham's salt was obtained by adding Prepare 50 grams of CaO to 0.355 grams of Graham's Salt and mix.

Example 2

The mass given in Example 1 was 100 ml Diluted water at room temperature with stirring

Example 3

A mass containing 1 part of calcium to 0.1 part of the P _n O (3n + i) component in sodium pyrophosphate was prepared by adding 6.24 g of sodium pyrophosphate to 50 g of CaCO ₃ .

Example 4

100 ml of water was added to the mass of Example 3 and mixed therewith.

Examples 5 to 29

In each of Examples 5 to 29, 80 g of water were mixed in a beaker with 10 g of lime or limestone, as indicated in column 1 of Table I below, the temperature of each mixture was adjusted to the temperature indicated for this in column 3 of Table I and the compound listed in column 4 of table I was added to each mixture in such an amount that the weight ratio of Ca to Graham's salt or P _n Op _{n +} 1) in the compound 1 used according to column 4 compared to that given in column 5 of table I. Number was, except that when O is given in column 4 of Table I, neither Graham's salt nor a compound of the formula I were used in the respective example.

Sulfur dioxide gas was bubbled into each mixture with stirring until pH 5.5 for them Examples was reached in which Ca (OH) 2 was applied, or until pH 3.0 was reached for such examples, where CaCO3 was used. The contents of each beaker were stirred and poured into a single one Transfer measuring cylinder of 100 ml and leave to stand for 2 hours. The "initial settling speed" and "volume finally settled" were measured in the above manner in each example and the results are shown in columns 6 and 7 of Table I, respectively.

The abbreviation »G. S. «was used in the tables below for Graham's salt and the Specification -O- under the connection in the tables denotes a comparative example for comparative purposes, wherein neither Graham's salt nor a compound of the formula I was used in this example.

Table I.

example . Ca- 7 ¹⁰ C link Weight Initial settling In the end No. source relationship speed deposed volume ml / min ml 5 Ca (OH) ₂ 2 -O- O 0.69 42 6th as well as well G. S. 0.002 0.75 39 7th as well 90 -O- O 0.26 59 8th as well as well G. S. 0.09 1.8 38 9 as well 25th -O- O 0.38 53 10 as well as well G. S. 0.04 4.2 21 11th as well as well -O- O 0.38 53 12th as well as well Na ₂ P ₂ O ₇ · 10H ₂ O 0.012 1.6 34 13th as well as well as well 0.20 2.4 65 14th CaCO ₃ as well -O- 0 0.3 42 15th as well as well Na ₄ P ₂ O ₇ · 10H ₂ O 0.012 4.7 19th 16 as well 95 -O- 0 4.0 40 17th as well as well Na ₄ P ₂ O ₇ · 10H ₂ O 0.08 7.4 26th 18th as well 5 -O- 0 0.10 41 19th as well as well Na ₄ P ₂ O ₇ -IOH ₂ O 0.0012 0.12 15th 20th Ca (OH) ₂ 2 -O- 0 0.69 42 21 as well as well Na ₄ P ₂ O ₇ · 1OH ₂ O 0.08 2.5 35 22nd as well 90 -O- 0 0.26 59 23 as well as well Na ₄ P ₂ O ₇ -IOH ₂ O 0.08 2.1 31 24 CaCO ₃ 25th -O- 0 0.3 42 25th as well as well Na ₄ P ₂ O ₇ 0.01 6.4 32 26th Ca (OH) ₂ as well -O- 0 0.38 53 27 as well 27 Na ₅ P ₃ O ₁₀ 0.0013 0.73 48 28 as well 26th as well 0.027 6.4 21 29 as well 25th as well 0.13 7.6 37

B e i s ρ i e 1 e 30 to 37

The procedure in Examples 30 to 37 was the same as the procedure in the above Examples 5 to 29, but with after blowing

Table Π

of the sulfur dioxide gas into each mixture in air with stirring into each mixture for about 2 hours was blown in. Table II brings Examples 30 to 37 in the same way as Table I the Examples 5 to 29.

example Ca- r ° c link Weight Initial settling Finally gone Nn source relationship speed set volume ml / min ml 30th CaCO ₃ 5 -ο- 0 0.1 41 31 as well as well α s. 0.003 19.0 9 32 as well as well as well 0.2 3.2 20th 33 as well 95 -ο- 0 4.0 40 34 as well as well α s. 0.003 4.3 28 35 as well as well as well SY Λ-Ι
U ₁ WJ 11.0 π
ί ι 36 as well 25th -ο- 0 0.3 42 37 as well as well α s. OJ03 24.0 16

Examples 38-43

In each of Examples 38 to 43, 80 g of water were adjusted to the temperature given in column 3 of Table III for each example, and sufficient Graham's salt was added to each water so that the weight ratio of Ca to Graham's salt in each example 1 to that was the number given in column 5 of Table III; 10 g of finely divided CaCÜ3 were added to each water with mixing and SO ₂ gas was introduced into each water with stirring and 1 minute after the "beginning of the SO ₂ introduction and during the 9 minutes of additional introduction of SO ₂ , 2, 02 g of 50% aqueous H ₂ SO _{4 was added} to each water in an amount of two drops / 30 seconds. The SO ₂ disappeared when pH 3.0 was reached. The resulting slurries were separated with air for 2 hours rinsed, mixed and transferred to separate 100 ml graduated cylinders and allowed to stand for 2 hours The values for "initial settling rate" and "final settled volume" are listed for each example in Table III below

ίο

Table III

example Ca- rc link Weight Initial Finally gone 25th No. source relationship Initial settling set volume 43 speed 24 ml / min ml 38 CaCO ₃ 2 -ο- 0 1.25 35 39 as well as well α s. 0.003 1.81 32 40 as well 95 -O- 0 Solidified before completion 41 as well as well G. S. 0.2 3.1 42 as well 25th -ο- 0 0.57 43 as well as well α s. 0.03 62.0

Examples 44-48

In each of Examples 44 to 48, 80 g of water were adjusted to the temperature given for each example in column 3 of Table IV below. Graham's salt was added to each water in sufficient amount so that the weight ratio of Ca to Graham's salt used in each example was 1 to the number given in column 5 of Table IV. 10 grams of finely divided Ca (OH) _{2 was} added to each aqueous solution of Graham's salt. _{SO 2 was} blown into each solution for 15 minutes and, 1 minute after the start of the introduction of SO ₂ , the addition of 1.547 g of a 95.7% strength H ₂ SO ₄ to each solution was started. H ₂ SO-I was added over 4 minutes at a rate of one drop / 30 seconds and then at one drop / minute over 11 minutes. The resulting slurries showed pH 5.5. Each slurry obtained was mixed, placed in a single measuring cylinder of 100 ml, and allowed to stand for 2 hours. The results are given in Table IV below.

Tabel IV rc link Weight Initial Finally gone example Ca- relationship Initial settling set volume No. source speed ml / min ml 25th -ο- 0 0.44 51 44 Ca (OH) ₂ as well α s. 0.002 1.51 35 45 as well as well as well 0.02 2.53 29 46 as well 95 -ο- 0 0.08 72 47 as well as well α s. 0.2 0.90 40 48 as well

B e i s ρ i e 1 e 49 and 50

In each of Examples 49 and 50, 80 g of water at 25 ° C. in a beaker was sufficient Graham's salt mixed so that the weight ratio of Ca to that used in each example Graham's salt 1 to the number given for Graham's salt in column 5 of Table V was 1 g Fly ash from a technical coal incinerator

Table V

Elektrozitätsgemeindewerk and 10 g finely-divided | Ca (OH) _{2 was} added to each beaker with mixing and SO ₂ -GaS was bubbled in until the resulting slurries showed pH 5.5. Each slurry was transferred to a 100 ml graduated cylinder and allowed to stand for 2 hours. The results are given in Table V below.

example
No. Ca-
source rc link Weight
relationship Initial
Initial settling
speed
ml / min Finally gone
set volume
ml 49
50 Ca (OH) ₂
as well 25th
as well -ο-
α s. 0
0.04 0.40
3.06 53
24

B e i s ρ i e 1 e 51 and 52

In each of Examples 51 and 52 were dissolved 10 g of NaHSO3 in 90 ml water at 6O ⁰ C in a beaker and it was sufficiently added Graham's salt, the weight ratio of Ca to Graham's salt at 1 to the number of the column 5

to bring Graham's salt given in Table VI below, whereupon 10 g of Ca (OH) _{2 were} added and the mixture was stirred for 20 minutes. The resulting slurries were poured into individual measuring cylinders from

Table VI

Transferred 100 ml and left to stand for 2 hours. The results are in Table VI below contain.

example
No. Ca-
source rc link Weight
relationship Initial
Initial settling
speed
ml / min Finally gone
set volume
ml 51
52 Ca (OH) ₂
as well 60
as well -O-
GS 0
0.04 5.2
5.9 44
35

Examples 53 to 55

In each of Examples 53 to 55, 80 g of water at 25 ^° C. were placed in separate beakers. Sufficient Graham's salt was added to each beaker with mixing to achieve a weight ratio of Ca to Graham's salt of 1 to the number given in column 5 of Table VI below for Graham's salt, followed by 0.53 g of MgO each with mixing and then 10 g of Ca (OH) _{2 were added to} each with mixing. SO ₂ gas was blown in with stirring until the resulting slurries reached pH 5.5. The slurries were placed in separate 100 ml graduated cylinders and allowed to stand for 2 hours. The results are given in Table VII below.

Tabel VII rc link Weight
relationship Initial
Initial settling
speed
ml / min Finally gone
set volume
ml example
No. Ca-
source 25th
as well
as well -ο-
σ. s.
as well 0
0.018
0.037 0.43
2.13
4.85 54.5
28
20th 53
54
55 Ca (OH) ₂
as well
as well

Claims (2)

Patent claims: 1. A method for removing sulfur oxides from gases, the gases being ^{contacted at 2 to 100 0} C with an aqueous scrubbing liquid which contains calcium ions, sulfite ions or bisulfite ions and alkali metal polyphosphate, characterized in that the alkali metal polyphosphate is Graham's salt or one of the general formula M ₁ P _n O _{13n +} I) is used, the weight ratio of Ca to Graham's salt or Ca to the P _n Of _{3n +} ir portion of M ₁ P _n Op _{n +} υ having a value of 1: 0.0015 to 0.2 , M a cation, π a positive integer greater than 1, χ the value (n + 2) / yundyd] e denotes the valency of M. 2. The method according to claim 1, characterized in that the components are contacted at 2 to 60 ° C, and a weight ratio of Ca to Graham's salt or Ca to the P _n O (3 "+ υ proportion of 1: 0.01 up to 0.04 is observed.