Magnesium-Enhanced Sulfur Dioxide Scrubbing With Gypsum Formation

The present invention relates to a method of removing sulfur dioxide from hot flue gases, and more specifically to a magnesium-enhanced lime scrubbing method for removing sulfur dioxide from a gaseous stream which produces gypsum as a by-product.

The use of lime to remove sulfur dioxide from gaseous streams, such as combustion gas streams from combustion systems of power plants, is a proven technology which uses a wet scrubbing unit. Generally, an aqueous slurry of lime is charged to a wet scrubbing unit for countercurrent flow to a flue gas stream passing through the wet scrubbing unit, with formation of calcium sulfite and calcium bisulfite through reaction of the lime with the sulfur dioxide present in the flue gas. A major improvement in early such lime scrubbing processes was the provision of an effective amount of magnesium ions in the aqueous scrubbing slurry that significantly improved the efficiency of such processes. The magnesium-enhanced lime scrubbing processes, as described, for example, in U.S. Pat. Nos. 3,919,393 and 3,919,394 to Joseph Selmiezi, have become commercially accepted. In such magnesium-enhanced lime scrubbing processes for sulfur dioxide removal, the solids content of the aqueous scrubbing medium in the wet scrubbing unit is normally maintained with a range of about 3-10 percent by weight. Such magnesium-enhanced lime scrubbing processes normally produce a large supply of calcium sulfite which has either been disposed of in sludge ponding or has in one manner or another been transformed into a useful by-product, such as gypsum. In U.S. Pat. No. 4,976,936, to Ronald J. Rathi and Lewis B. Benson, for example, which is assigned to the assignee of the present invention, a calcium sulfite-containing aqueous discharge from a magnesium-enhanced lime scrubbing process is passed to a thickener and an aqueous sludge from the thickener is passed to a mixing tank where sulfuric acid is added to dissolve the calcium sulfite and the resultant solution is oxidized to produce a gypsum product that is separated from the aqueous media. Also, as described therein, if desired, a portion of the discharge from the scrubber may be passed directly to the mixing tank, by-passing the thickener, and mixed with sulfuric acid prior to oxidation. In such a process, a large scale thickener is required to handle the aqueous slurry discharged from the wet scrubbing unit and sulfuric acid must be added to the mixer.

The present invention is a method of operating a magnesium-enhanced lime scrubbing process for sulfur dioxide removal from flue gases while producing gypsum and optionally magnesium hydroxide by-products.

An aqueous slurry of magnesium-enhanced lime scrubbing medium is contacted with a flue gas containing sulfur dioxide in a wet scrubbing unit and the solids content of the aqueous slurry maintained at an amount of between 13 to 20 percent by weight. The solids produced will be primarily calcium sulfite with some other residual solids, while magnesium sulfite and magnesium sulfate present are in solution. A bleed stream of the aqueous slurry is removed from the recirculating liquor in the wet scrubbing unit and associated hold tank, at a location after contact of the slurry with the gases but prior to passage to the hold tank, and is passed directly to an oxidizing unit. In the oxidizing unit, the calcium sulfite present in the bleed stream is oxidized to gypsum (calcium sulfate). The gypsum and residual solids present are removed from the aqueous slurry to produce a clarified solution or aqueous liquor, which clarified aqueous liquor is returned to the wet scrubbing unit and mixed with the aqueous slurry therein.

By removing a portion or bleed stream of the aqueous slurry from the wet scrubbing unit after contact with the flue gases but prior to passage to the hold tank, the portion will be at a pH of about 4.5-5.6 and can be oxidized in the oxidizing unit without the need to add sulfuric or other acidic compounds to the oxidizing unit.

The clarified aqueous liquor resulting from a removal of the gypsum and residual solids may optionally be treated to produce a magnesium hydroxide salable by-product. The clarified aqueous liquor, containing dissolved magnesium sulfate, may be contacted with a lime slurry such that a reaction precipitates gypsum and provides a magnesium hydroxide suspension that is separated, clarified and collected for use or sale.

The invention will become more readily apparent from the following description of a preferred embodiment thereof shown, by way of example only, in the accompanying drawing which is a flow diagram illustrating a present preferred method of the present invention.

The present invention provides a magnesium-enhanced lime scrubbing method for removing sulfur dioxide from a gaseous stream while producing gypsum as a by-product and optionally also providing magnesium hydroxide as a by-product.

The magnesium-enhanced aqueous lime scrubbing medium used in the present method is a lime slurry containing magnesium ions such as scrubbing slurries described in the previously mentioned U.S. Pat. No. 3,919,393, U.S. Pat. No. 3,919,394, U.S. Pat. No. 3,914,378 and U.S. Pat. No. 4,976,936, all of which are incorporated by reference herein. The magnesium ion content of the aqueous scrubbing medium in the present process would be between about 2500-15000 parts per million (ppm).

Referring now to the drawing, the present method is schematically illustrated, showing a wet scrubbing unit 1 to which a sulfur dioxide-containing gas, such as a combustion flue gas, is charged through line 3. An aqueous scrubbing medium, in the form of an aqueous slurry of magnesium-enhanced lime is added to a hold tank 5 through line 7, and aqueous scrubbing slurry is passed through line 9 from the hold tank 5 by means of pump 11 to spray nozzles 13 in the wet scrubbing unit 1. The aqueous slurry of magnesium-enhanced lime contacts the flue gas in the wet scrubbing unit 1 to remove sulfur dioxide therefrom, with clean gas discharged from the wet scrubbing unit 1 through discharge line 15. Fresh water, if desired, may be added to the wet scrubbing unit 1 through line 16 to wash demister units (not shown) at the upper region of the wet scrubbing unit 1. In accordance with the present method, the solids content of the aqueous scrubbing slurry in the wet scrubbing unit 1 is permitted to rise to a level of between 13-20 percent by weight, which solids comprise primarily calcium sulfite (primarily calcium sulfite hemihydrate: CaSO.sub.3.1/2H.sub.2 O), with magnesium sulfite and magnesium sulfate in solution. The pH of the aqueous scrubbing slurry in the wet scrubbing unit is maintained at a pH of between 4.5 to 5.6, preferably between 5.0 to 5.5, while the pH of the medium in the hold tank 5, to which effluent from the wet scrubbing unit 1 is passed, will be at a slightly higher value, such as between about 5.5-6.0 due to the addition of magnesium-enhanced lime slurry through line 7.

A bleed stream of aqueous slurry is removed from the wet scrubbing unit 1 through line 17, which bleed stream is at a pH of between 4.5 to 5.6, and preferably between 5.0 to 5.5, and contains 13-20 percent by weight of calcium sulfite solids along with dissolved magnesium sulfite and magnesium sulfate. This bleed stream is passed through line 17 directly to an oxidizing unit 19. An advantage of operating the wet scrubbing unit at a solids content of between 13-20 percent by weight and removing the bleed stream with such a solids content is the formation in the aqueous scrubbing slurry of very fine CaSO.sub.3.1/2H.sub.2 O particles therein. These very small particles (about 5 microns vs normal solids sizes of about 12-20 microns) will dissolve more readily in the oxidizing unit. In the oxidizing unit 19, air, which may be oxygen-enriched, is charged through line 21, so as to oxidize calcium sulfite to calcium sulfate and magnesium sulfite to magnesium sulfate, with the solids content of the aqueous slurry increasing to about 17-26 percent by weight of calcium sulfate, and with the magnesium sulfate dissolving in the aqueous medium. The oxidized aqueous slurry is discharged from the oxidizing unit 19 through line 23, and by means of pump 25 is fed to a solids separator 27 such as a hydroclone, where gypsum and any residual solids are removed and discharged through line 29, while a clarified liquor produced is discharged through line 31. The clarified liquor from line 31, which may contain about 1-2 percent by weight of fine solids is passed to a fines separator 33. Fine solid material separated in the fines separator 33 is discharged through line 35 as a waste material, while the clarified solution resulting from the separation therein is fed through line 37 to a supply of make-up process water tank 39 for use in the wet scrubbing unit 1. The clarified solution from make-up process water tank 39 is returned to the wet scrubbing unit 1 by means of pump 41 in line 43, to the wet scrubbing unit 1 by means of pump 41 in line 43, preferably by adding the same to the hold tank 5.

In order to provide further make-up process water to the wet scrubbing unit 1, the gypsum in line 29 from the solids separator 27, which will normally be a wet solid component containing about 60-75 percent by weight of water, is filtered on a filter 45, with filtrate passing through line 47 to the make-up process water tank 39, while the filter cake of gypsum is removed through line 49 for collection.

A benefit of the present method is the ability to collect magnesium from the clarified solution as magnesium hydroxide which may be used in the wet scrubbing unit or may be sold as a valuable by-product. As illustrated, clarified solution, or a portion thereof, fed to the make-up process water tank 39 may be discharged through line 51 and, by means of pump 53, fed to a regeneration tank 55. In the regeneration tank 55, magnesium sulfate contained in the clarified solution is reacted with a lime slurry fed through line 57 so as to produce a precipitated gypsum solid and magnesium hydroxide. The reacted material from regeneration tank 55 is fed through line 59, by means of pump 61, to a further solids separator 63, such as a hydroclone, where gypsum solids are separated and discharged through line 65, while a resulting magnesium hydroxide aqueous suspension is passed through line 67 to a magnesium hydroxide separating unit 69. In the magnesium hydroxide separating unit 69, magnesium hydroxide is separated from the resultant aqueous medium and discharged through line 71 for re-use or sale, while the aqueous medium is passed through line 73 to the make-up process water tank 39 for use in the wet scrubbing unit 1.

The present method has distinct advantages over prior art processes where oxidation of calcium sulfite from a magnesium-enhanced lime scrubbing process has been effected. A primary example is the fact that a large volume thickener or solids separator is not required to concentrate a calcium sulfite sludge that is subsequently oxidized, such as is used in U.S. Pat. No. 4,976,936. With use of the hydroclone, only a small thickener or clarifying tank is required to remove fines or residual solids from the aqueous liquor. Another advantage is the avoidance of dilution or recirculation of acidic liquors to the oxidizing unit. Since the bleed stream from the downcomer is at a pH of 4.5-5.6, about three-fourths of the sulfite ions (SO.sub.3.sup.=) in solution in the bleed stream would be in the form of HSO.sub.3.sup.- which would readily oxidize to H.sub.2 SO.sub.4 :

such that there will be sufficient acid produced in situ to neutralize any calcium carbonate that may be formed at the pH of about 6 in the wet scrubber. The acid so produced will dissolve the calcium carbonate and ultimately allow the digestion of the CaSO.sub.3 in the oxidizing unit.

A tight water balance is maintained in the system such that a high concentration of dissolved salts, principally magnesium salts, is maintained in the wet scrubbing unit. As an optional step, the separated, wet, gypsum solids from line 65 may be returned, along with residual water, to the wet scrubbing unit 1. These gypsum solids will provide seed crystals for oxidation, following subsequent removal in the bleed stream, in the oxidizing unit 19, and will carry some residual magnesium hydroxide (about 1-2 percent by weight) that supplements the magnesium in the magnesium-enhanced lime added to the scrubbing system. These returned gypsum solids will then be substantially removed with the gypsum solids removed through line 49.

As described, the present method provides a magnesium-enhanced lime scrubbing method to remove sulfur dioxide from a gaseous stream and produces gypsum, while recovering magnesium hydroxide, while using low volume settling devices for ready removal of the system solids.