CA2133694C - Process for producing magnesium sulfite hexahydrate in a flue gas desulfurization system - Google Patents

Process for producing magnesium sulfite hexahydrate in a flue gas desulfurization system

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Publication number
CA2133694C
CA2133694C CA 2133694 CA2133694A CA2133694C CA 2133694 C CA2133694 C CA 2133694C CA 2133694 CA2133694 CA 2133694 CA 2133694 A CA2133694 A CA 2133694A CA 2133694 C CA2133694 C CA 2133694C
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Prior art keywords
magnesium
sulfur dioxide
gaseous stream
magnesium sulfite
sulfite
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CA 2133694
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French (fr)
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CA2133694A1 (en
Inventor
Daniel W. Berisko
John W. College
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Dravo Lime Co
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Dravo Lime Co
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Abstract

A process for removing sulfur dioxide from a flue gas stream where the magnesium sulfite removed from the effluent of a wet scrubber is in the form of magnesium sulfite hexahydrate. The chemistry of the aqueous scrubbing medium is controlled with specified ranges of magnesium ion content, sulfite ion content and pH. A portion of the aqueous solution removed from the scrubber is cooled, and solids crystallized therefrom, in the form of magnesium sulfite hexahydrate.

Description

~369~

PROCESS FOR PRODUCING MAGNESIUM SULFITE
HEXAHYDRATE IN A FLUE GAS DESULFURIZATION SYSTEM
Fleld of the Inventlon The present lnvention relates to an improved flue gas desulfurlzatlon system where calclum and magnesium scrubblng components are used to remove sulfur dloxide from a gaseous stream, and more speclflcally to such a system where magneslum sulflte is produced and recovered.
Backqround on the Inventlon The use of magneslum-enhanced lime scrubblng processes to remove sulfur dioxlde from flue gases, such as those from fossll fuel burnlng power plants, has been well accepted commerclally. In such scrubblng processes, an alkaline earth metal component, such as llme, wlth a magneslum component, such as magneslum oxide or hydroxlde, ls contacted with the flue gases ln a wet scrubber. Such processes are generally descrlbed in US 3,919,393, US
3,919,394, US 3,914,378 and US 4,976,937. all of whlch are asslgned to the assignee of the present lnventlon. As descrlbed ln the latter patent, US 4,976,937, sulfur, polysulfides or other thiosulfate (S2O3 ) precursors can be added to the scrubblng ll~uor ln order to act as a free radlcal scavenger so as to lnhlblt the oxldatlon or sulflte produced ln the scrubblng process to sulfates.

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The magnesium-~nh~nc~ lime scrubbing processes are typically characterized by high levels of dissolved magnesium and sulfites. The sulfite levels are increased by the suppression of natural oxidation of sulfites to sulfates such as by the addition of sulfur or other thiosulfate precursors. Sulfur reacts with sulfites to produce thiosulfates. Such sulfite oxidation inhibition shifts the scrubbing liquor chemistry from subsaturated with respect to MgSO3 to a MgSO3 supersaturated mode.
Supersaturation for MgS03 may also be achieved without sulfur addition if a high magnesium oxide or hydroxide level is provided.
The magnesium sulfite produced in a magnesium enhanced lime scrubbing process for removal of sulfur dioxide from flue gases is usually found as predominantly magnesium sulfite trihydrate (MgSO3-3H20), with a minor amount of magnesium sulfite hexahydrate (MgS03 6H20). The magnesium sulfite trihydrate is usually formed as small crystals which are difficult to dewater and can provide disposal problems. Magnesium sulfite hexahydrate, on the other hand, is usually formed as large crystals and are more easily separated and dewatered.
Ma~nesium sulfite does not exist, at present, as a chemical commodity of any appreciable volume. The primary large scale commercial application for magnesium sulfite is in the magnesium bisulfite pulping process as practiced by the pulp and paper industry. These are, however, in-situ, closed-loop processes in which elemental sulfur is burned to produce sulfur dioxide gas which is then absorbed by a magnesium hydroxide slurry in a 3 !) 9 ~1 countercurrent absorber to produce magnesium sulfite. At the pulping digester pH of about 4.5, the sulfite is primarily in the bisulfite form due to equilibrium reactions. Once the digestion process is complete and the~paper pulp product and the by-product magnesium lignosulfonate are removed, the "strong red liquor"
process slurry is directed to an evaporator where the magnesium bisulfite is decomposed to sulfur dioxide which reports back to the absorber, and magnesium oxide which is recovered and slaked to magnesium hydroxide and returned to the absorber. There are losses of both magnesium and sulfur dioxide in such processes, primarily in the magnesium lignosulfonate by-product, but also in the sulfur dioxide absorber, wastewater, and recovery furnaces. These losses must be recovered by using make-up reagents, namely elemental sulfur, magnesium oxide, and/or magnesium hydroxide.
These reagents are all expensive, prone to market price fluctuations, and in addition, the processes required to incorporate them into the bisulfite pulping process are energy intensive and require careful control.
It is an object of the present invention to produce magnesium sulfite in the hexahydrate form, from a magnesium-enhanced lime scrubbing process, that may be used in bisulfite pulping processes and other commercial processes.

SUMMARY OF THE lNv~NllON
The present process provides for the production of magnesium hexahydrate in a wet scrubbing process for removing ~ 1 3 ~, ~ v~ !~

sulfur dioxide from a flue gas using a magnesium-enhanced lime. A
gaseous stream is contacted with an aqueous scrubbing medium containing calcium and magnesium scrubbing components such that sulfur dioxide in the gases is converted to magnesium sulfite. The magnesium sulfite produced is primarily magnesium sulfite trihydrate with a small amount of magnesium sulfite hexahydrate present. As is conventional, a bleed stream or portion of the magnesium sulfite-containing aqueous solution is discharged from the wet scrubber.
In accordance with the present invention, the aqueous solution containing magnesium scrubbing components is maintained such that a magnesium ion content of between about 5,000 - 12,000 parts per million (ppm) is present, a sulfite content of between about 3,000 - 18,000 ppm is present, and a pH of between about 6.0 - 7.0 is present, so as to produce an aqueous scrubbing solution supersaturated with respect to magnesium sulfite, which also contains some suspended solids. A portion of the supersaturated aqueous scrubbing solution is withdrawn from the scrubber and suspended solids removed therefrom. After the suspended solids are removed, the portion of supersaturated aqueous scrubbing solution is treated, by lowering the temperature thereof, and optionally adjusting the pH thereof above 7.0 to about 7.5. The temperature of the portion of supersaturated aqueous scrubbing solution is generally about 110 - 120~F (45 - 50~C) before treatment. The treatment of the portion of supersaturated aqueous scrubbing solution causes a conversion of magnesium sulfite trihydrate 2 1 3 ~

therein to magnesium sulfite hexahydrate, which magnesium sulfite hexahydrate, so produced, crystallizes and is separated.
In an alternative embodiment of the present process, a portion of the supersaturated aqueous scrubbing solution may be passed directly to a cooler-crystallizer to convert magnesium sulfite trihydrate to magnesium sulfite hexahydrate and the suspended solids separated, along with the aqueous media, from the magnesium sulfite hexahydrate produced.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be further understood by reference to the attached drawings showing an embodiment thereof, where:

Figure 1 is a schematic representation of a preferred embodiment of the present process;

Figure 2 is a graphical representation showing a predominance of magnesium sulfite hexahydrate form as a stable species at lower temperatures vs the predominance of magnesium sulfite trihydrate form at elevated temperatures in an aqueous medium;

Figure 3 is a graphical representation of the pH effect on the saturation value of magnesium sulfites relative to bisulfites in an aqueous medium; and 2 ~ 3 3 ~ ~ ~

Figure 4 is a schematic representation of another embodiment of the present process.

DE~ATT~n DESCRIPTION

The present process enables the recovery of magnesium sulfite solids from a magnesium-enhanced lime scrubbing process for removing sulfur dioxide from flue gases where the magnesium sulfite resulting is in the form primarily as magnesium sulfite hexahydrate.
Referring now to Figure 1, the process is illustrated, showing a wet scrubbing unit 1 to which a sulfur dioxide-containing gas is charged through line 3 and clean gas discharged through line 5. An aqueous scrubbing medium conta;n;ng calcium and magnesium scrubbing components is charged to the wet scrubbing unit 1 through line 7 and contacts the sulfur dioxide-containing gas to remove sulfur dioxide therefrom. The aqueous scrubbing medium is recycled through the wet scrubbing unit 1 by means of recycle line 9. The aqueous scrubbing medium is a lime slurry containing magnesium ions as described in the aforementioned US 3,919,393, US 3,919,394, US
3,914,378 and US 4,976,937. Also, as described in US 4,976,937, a reductant, such as sulfur, a polysulfide, a thiosulfate, or a thiosulfate precursor is added to the scrubbing liquor in the wet scrubbing unit through line 11, so as to provide high levels of dissolved magnesium and sulfites in the aqueous scrubbing liquor, by suppressing oxidation of such sulfites to sulfates. Such oxidation inhibition shifts the aqueous medium chemistry from 2 1 .3 ~, ~ 3 i-l subsaturated with respect to magnesium sulfite to a supersaturated mode. Either a thiosulfate compound or a thiosulfate precursor is preferred as the reductant or oxidation inhibitor and is added in an amount such as will preferably provide about 0.5 to 40 millimoles of thiosulfate per liter of aqueous medium contained in the wet scrubber.
The aqueous scrubbing medium which is used to contact the sulfur dioxide containing gas should be maintained so as to have a magnesium ion content of between 5,000 - 12,000 ppm, preferably about 6,000 - 8,500 ppm, a sulfite ion content of between 3,000 -18,000 ppm, preferably between about 5,000 - 12,000 ppm, and a pH
of between 6.0 - 7Ø In order to obtain the magnesium ion and sulfite ion content required, generally either a high magnesium content lime would be required, or a reductant as mentioned hereinbefore would need to be added to the scrubbing medium to reduce oxidation of sulfites to sulfates.
Generally, in such wet scrubbing processes, the temperature of the aqueous scrubbing medium and within the scrubber is about 50~C. The aqueous scrubbing medium, after contact with the sulfur dioxide will contain magnesium sulfite in the form of a major amount of magnesium sulfite trihydrate and a minor amount of magnesium sulfite hexahydrate and magnesium bisulfite. The aqueous medium will thus comprise a solution of magnesium sulfite containing some suspended solids, primarily CaS03 ~ ~ H20 and CaS04 ~ ?~ H20.

~ ~ 3~3 .fj .'3 ~
-According to an embodiment of the present invention, a portion of the supersaturated aqueous scrubbing solution is withdrawn from the wet scrubbing unit 1 through line 13 and fed to a clarifier or thickener --15, where the suspended solids are separated from the supersaturated aqueous scrubbing solution.
Thickened liquor, containing the suspended solids is ~;~ch~rged from the thickener 15 through line 17, while the supersaturated aqueous solution is passed from line 19 to a feed tank 21. The suspended solids are removed so as to prevent scaling of later components used in the process and to assure a high purity magnesium sulfite hexahydrate production. From the feed tank 21, the supersaturated aqueous solution is passed through line 23 to a cooler 25 and then through line 27 to a crystallizer 29. In some instances, the cooler 25 and crystallizer 29 could be a combined piece of equipment.
By reference to Figure 2, it can be seen that the relative saturation value of the sulfites, magnesium sulfite trihydrate and magnesium sulfite hexahydrate, shift relative to the temperature of a solution thereof. As shown, for example, at a pH
of 7.0 and at a temperature of about 50~C, the magnesium sulfite trihydrate (line A) saturation will be about 1.3 or supersaturated, while the magnesium sulfite hexahydrate saturation (line B) will be about 0.7 or subsaturated. Upon cooling of the solution to about 25~C however, the magnesium sulfite hexahydrate saturation increases to about 1.6, or supersaturated, while the magnesium sulfite trihydrate saturation falls to about 0.7 or subsaturation.

Also, as shown, at a pH of 6.5 the saturation value of magnesium sulfite trihydrate (line C) is about 1.0 at 50~C and about 0.6 at 25OC, while that of magnesium sulfite hexahydrate (line D) is about 0.5 at 50~C and about 1.4 at 25~C.
After cooling of the supersaturated aqueous scrubbing solution, from the scrubber temperature of 45 - 50~C to a temperature of between about 15 - 40~C and preferably to about 25~C
or below, in the cooler 25, the supersaturated aqueous solution contains magnesium sulfite hexahydrate in stable form as the predominant sulfite, and the solution is passed to the crystallizer 29, which may be any suitable commercially available crystallizer.
Upon crystallization of the magnesium sulfite hexahydrate from the aqueous solution thereof, the aqueous medium and crystals are discharged from the crystallizer 29 through line 31 to a dewatering device 33. The water is separated in dewatering device 33 and discharged through line 35 while the dewatered magnesium sulfite hexahydrate crystals are passed through line 37 to a dryer 39 where they are dried, with resultant water vapor discharged through line 41 while the dry magnesium sulfite hexahydrate crystals are passed through line 43 to a collection device 45 as end product. The drying of any surface moisture from the magnesium sulfite hexahydrate crystals is important since, although such crystals are stable, any entrained or surface moisture associated with the crystals (MgS03 solution) will quickly oxidize to magnesium sulfate (MgSO4) creating a white scale on surface of the otherwise clear rhombic crystals of magnesium sulfite hexahydrate. Such sulfates ~ ~ 3 ~

.

could interfere with prospective uses of the product. The dried magnesium sulfite hexahydrate crystals should be maintained in air-tight containers for storage and shipping.
In an optional use of the magnesium sulfite hexahydrate, the dewatered magnesium sulfite hexahydrate from line 37 could be diverted through line 47 to a roaster 49 where the crystals can be heated to produce magnesium oxide (MgO) as a salable product which is discharged through line 51 and sulfur dioxide ( S~2 ) which is discharged through line 53 for uses such as in the formation of sulfuric acid.
In another embodiment of the process of the present invention, the portion of supersaturated aqueous scrubbing solids, after removal of the suspended solids in thickener 15 is passed through line 19 to the feed tank 21 and the pH of the solution is increased to a value of between 7.0 to 7.5 by an addition of an alkaline reagent, such as lime, from a source 55 through line 57.
The effect of an increase in pH on the saturation values of magnesium sulfite is shown in Figure 3. If the pH of the supersaturated aqueous solution is about 6.0, increase to about 7.0 may be sufficient to crystallize out magnesium sulfite hexahydrate, or if the pH of the supersaturated aqueous solution is between about 6.5 - 7.0, an increase to about 7.5 may be necessary to assist in crystallizing out a sufficient amount of magnesium sulfite hexahydrate.
The effect of pH on the percent sulfites, a crystallizable moiety, as opposed to bisulfites which remain in solution is shown in Figure 3. As shown, at a pH of about 5.0, only about 8 percent of the sulfites in a solution will be in the form of So3= ions, while the remainder will be HS03- ions, while with an increase in pH to about 7.0, about 90% of the sulfites will be in the form of S03= ions, while only about 10% will be in the form of HS03- ions. Thus, increasing the pH of a sulfite ion containing solution increases the sulfites present and thus would provide an increased yield of magnesium sulfites in the present process.
The aqueous media, after addition of a base to raise the pH, is then passed through line 23 to cooler 25 and thence through line 27 to crystallizer 29 for crystallization and subsequent separation and drying of the magnesium sulfite hexahydrate produced.
As an example of the present process, a magnesium-containing lime scrubbing process was carried out using generally the process described in US 3,919,393, with sulfur added to the lime slurry according to the teachings of US 4,976,937 to provide a thiosulfate content in the scrubbing liquor of about 2,000 ppm.
The scrubbing liquor was maintained so as to have a magnesium ion content of between about 6,000 - 8,500 ppm and a sulfite ion content of between about 8,000 - 16,000. A portion of the supersaturated aqueous scrubbing solution was removed, clarified, and cooled, with solids crystallizing from the solution. The contents of the operating scrubber liquor and the cooled scrubbing liquor were analyzed and the results were found to be:

f i 33694 Scrubber Crystalllzing Operatlng Area Temp ~C 50 20 pH 6.0 6.0 Calclum (mg/L) 88 88 Mg (mg/L) 6Z62 6262 Cl (mg/L) 6400 6400 SO4 (mg/L) 8506 8506 so3 (mg/L) 15412 15412 R.S. for MgSO3-3H2O1.21 0.69 R.S. for MgSO3-6H2O0.73 2.13 R.S. = 1 for solution to be saturated.
The value R.S. is used to ldentlfy the relative saturatlon of the solutlon with a partlcular sulflte and, as shown, at the 20~C crystallizlng area the solutlon was very supersaturated for MgSO3-6H2O whlch crystalllzed out from the solutlon. The magneslum sulflte sollds that separated from the cooled solutlon were determlned by analysls to be 96.08 percent by welght MgSO3-6H2O and only 1.01 percent by welght MgSO3-3H2O.
In the embodlment of the present process lllustrated ln Flgure 4, a portlon of the supersaturated aqueous scrubblng solutlon wlthdrawn from the wet scrubblng unlt 1 through llne 13 ls divlded, with a portlon thereof passing through line 61 to a clarlfler or thickener 15. The other portlon ls passed by line 13 to a chlller-crystalllzer unlt 63 to whlch llme ls added from a source 65 through llne 67. After coollng of the supersaturated aqueous solutlon and crystalllzatlon of magneslum sulflte hexahydrate, the aqueous media and crystals are passed through llne 69 to a separator 71, such as a hydroclone or screen washer. In "~

~133~9l~

the separator 71, the larger sized magnesium sulfite hexahydrate is separated from the aqueous media and small sized suspended solids and passed through line 73 to a collector 75. The aqueous solution or media and suspended solids are then returned to the thickener 15 through line 77.
Thus, according to the present process, sulfur dioxide can be removed from a flue gas by contact with a magnesium-enhanced lime scrubbing medium with production and separation of magnesium sulfite hexahydrate as a useful by-product.

Claims (16)

1. In a process for removing sulfur dioxide from a gaseous stream wherein the gaseous stream is contacted with an aqueous scrubbing medium containing calcium and magnesium scrubbing components such that sulfur dioxide in the gases is converted to magnesium sulfite, in the form of magnesium sulfite trihydrate and magnesium sulfite hexahydrate, and removed therefrom with a portion of magnesium sulfite-containing aqueous solution discharged from the wet scrubber, the improvement wherein;
the aqueous solution containing magnesium scrubbing components is maintained so as to contain a magnesium ion content of between 5,000 - 12,000 ppm, a sulfite ion content of between 3,000 - 18,000 ppm, and at a pH of between 6.0 - 7.0, so as to produce an aqueous scrubbing solution supersaturated with respect to magnesium sulfite and containing suspended solids;
withdrawing at least a portion of the supersaturated aqueous scrubbing solution from the wet scrubber; and adjusting the temperature of the portion of supersaturated aqueous scrubbing solution from between about 45 - 50°C to between about 15 - 40°C convert magnesium sulfite trihydrate to magnesium sulfite hexahydrate and crystallize magnesium sulfite hexahydrate therefrom.
2. The process for removing sulfur dioxide from a gaseous stream as defined in claim 1 wherein a reductant is added to the aqueous scrubbing medium to prevent oxidation of sulfites to sulfates therein.
3. The process for removing sulfur dioxide from a gaseous stream as defined in claim 2 wherein the reductant is sulfur.
4. The process for removing sulfur dioxide from a gaseous stream as defined in claim 2 wherein the reductant is a polysulfide.
5. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 1 to 4 wherein the pH of the supersaturated aqueous scrubbing solution is increased prior to the adjusting of the temperature.
6. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 1 to 5 wherein the sulfite ion content is between 5,000 - 12,000 ppm.
7. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 1 to 6 wherein the magnesium ion content is between 6,000 - 8,500 ppm.
8. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 1 to 7 wherein the suspended solids are removed from the portion of supersaturated aqueous scrubbing solution prior to the adjusting of temperature.
9. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 1 to 8 wherein the suspended solids are separated from the magnesium sulfite hexahydrate after the crystallization.
10. In a process for removing sulfur dioxide from a gaseous stream wherein the gaseous stream is contacted with an aqueous scrubbing medium containing calcium and magnesium scrubbing components such that sulfur dioxide in the gases is converted to magnesium sulfite, in the form of magnesium sulfite trihydrate and magnesium sulfite hexahydrate, and removed therefrom with a portion of magnesium sulfite-containing aqueous solution discharged from the wet scrubber, the improvement wherein;
the aqueous solution containing magnesium scrubbing components is maintained so as to contain a magnesium ion content of between 5,000 - 12,000 ppm, a sulfite ion content of between 3,000 - 18,000 ppm, and at a pH of between 6.0 - 7.0, so as to produce an aqueous scrubbing solution supersaturated with respect to magnesium sulfite and containing suspended solids;
withdrawing at least a portion of the supersaturated aqueous scrubbing solution, at a temperature of between about 45 - 50°C, from the wet scrubber; and adjusting the temperature of the portion of supersaturated aqueous scrubbing solution to between about 15 - 40°C to convert magnesium sulfite trihydrate to magnesium sulfite hexahydrate and crystallize magnesium sulfite hexahydrate therefrom, wherein (1) the suspended solids are removed after the portion is withdrawn but before the temperature is adjusted, or alternatively, (ii) the withdrawn portion is first passed directly to a cooler-crystallizer and then the suspended solids are separated along with the aqueous medium from the magnesium sulfite hexahydrate produced.
11. The process for removing sulfur dioxide from a gaseous stream as defined in claim 10 wherein the pH of the supersaturated aqueous scrubbing solution is increased prior to the adjusting of the temperatures.
12. The process for removing sulfur dioxide from a gaseous stream as defined in claim 10 or 11 wherein the magnesium ion content is between 6,000 - 8,500 ppm and the sulfite ion content is between 5,000 - 12,000 ppm.
13. The process for removing sulfur dioxide from a gaseous stream as defined in claim 10, 11 or 12 wherein sulfur is added to the aqueous scrubbing medium as a reductant to provide thiosulfate ions therein.
14. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 10 to 13 wherein the suspended solids are removed from the portion of supersaturated aqueous scrubbing solution prior to the adjusting of temperature.
15. The process for removing sulfur dioxide from a gaseous stream as defined in any one of claims 10 to 14 wherein the suspended solids are separated along with the aqueous medium from the magnesium sulfite hexahydrate after the crystallization.
16. The process for removing sulfur dioxide from a gaseous stream as defined in claim 5 or 11, wherein the increase of the pH is conducted so that the resulting supersaturated aqueous scrubbing solution has a pH value of between 7.0 and 7.5.
CA 2133694 1993-10-25 1994-10-05 Process for producing magnesium sulfite hexahydrate in a flue gas desulfurization system Expired - Fee Related CA2133694C (en)

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US08/142,693 1993-10-25

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* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPH09122440A (en) * 1995-10-20 1997-05-13 Dravo Lime Co Method for scrubbing sulfur dioxide being accompanied with formation of pure product of magnesium sulfite
US6214313B1 (en) * 1999-04-13 2001-04-10 Dravo Lime, Inc. High-purity magnesium hydroxide and process for its production

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