CA2336436C - Process for producing gypsum from a calcium sulfite gas desulfurization slurry - Google Patents

Abstract

Calcium sulfate dehydrate (dehydrate gypsum) is produced from an aqueous flue gas desulfurization slurry containing primarily calcium sulfite. The aqueous slurry, containing calcium sulfite, is charged, at a pH of about 5.8 to 6.0 into the top of a vertically oriented pressurized oxidation vessel and passed downwardly therethrough while contacting the aqueous slurry with a countercurrent flow of oxygen-containing gas. The pressure in the bottom portion of the oxidation vessel is maintained at about 22-32 psig, and a pressure in the top portion of the oxidation vessel is maintained at about 10-17 psig. The aqueous slurry is maintained at a pH of about 4.0-5.5 and at a temperature of between about 180-217°F in the vessel such that calcium sulfite in the aqueous slurry is oxidized to dehydrate gypsum with the oxidized slurry removed from the bottom of the oxidation vessel.

Description

PROCESS FOR PRODUCING GYPSUM FROM A
CALCIUM SULFITE GAS DESULFURIZATION SLURRY
Field of the Invention The present invention relates to a process for producing gypsum (dehydrate gypsum: CaS04~2Hz0) from an aqueous flue gas desulfurization slurry containing calcium sulfite, in an efficient manner and with production of high purity gypsum.
Background of the Invention Wet scrubbing process for removing sulfur dioxide from sulfur dioxide-containing gases, such as gases from power plants or other coal combustion units, have been perfected which use lime or limestone as the scrubbing component in aqueous medium, with the production of aqueous waste sludge. The aqueous waste sludge contains primarily calcium sulfite, which is difficult to dewater and is often deposited into settling ponds for extended periods of time before solidification occurs.
Processes have been developed for the production of a useful byproduct from such calcium sulfite sludges, such as oxidizing the calcium sulfite to produce a useful calcium sulfate product such as gypsum.
In conventional processes where calcium sulfite sludges from a sulfur dioxide scrubbing process are oxidized to calcium sulfate, the slurries usually contain less than 25 percent solids.
Direct oxidation of a calcium sulfite slurry from a thickener of a desulfurization system generally requires dilution of the slurry solids content to reduce viscosity and improve oxidation (air bubble/slurry contact), with calcium sulfite slurries over 20 percent solids generally being so viscous as to slow oxidation. Limestone processes also generally require long residence times of 16-24 hours ~.n an oxidizing unit and an air/sulfite stoichiometric ratio of greater than 3:1 (moles oxygen used/moles oxygen required). T:n lirne oxidation pracesses, generally a solids content of between about 15-20 percent is required, with use of 2.5 to 3 hours residence tune and an air/sulfite staichiometric ratio of 2:1 to 3:1 far oxidation of the calcium sulfite to gypsum.
In U.S. 5,312,609, there is described a metrzod of removing sulfur dioxide from a gaseous stream with th.e production of alpha hernihydrate gypsum (aCaS04~?~HaO) as a byproduct. In the method described therein, a sulfur dioxide-containing gaseous stream is contacted with an aqueous scrubbing medium containing calcium and magnesium scrubbing components, in a wet scrubbing unit, with sulfur dioxide converted to calcium and magnesium sulfites. A
portion of the aqueous medium is remo°ved fx:°om the wet scrubbing unit and passed, at a sol~.ds ce~ntent of ~aetween about 5-35 weight percent to a pressurized oxidatiozz vessel. In the pressurized oxidation vessel, the calcium arzd magnesi~.xm :Pulfites are contacted with an oxidizing gas, at an elevated temperature of about 100 145°C (212-293°F) , a superatmosphex:i.c pressure of between about 20 60 pounds per square inch, and a pH of b~:;tween 2.5-5.5 to convert the calcium sulfite to alpha~hemihydrate ~yp~~um, and the magnesium sulfite present t.hexein to magnesium sv~.lfate. ~'he alpha hemihydrate gypsum precipitates fxom the aqueous medium while the magnesium sulfate remains in solution i.n the aqueous medium. After removal of the aqueous medium contair~.ing precipitated alpha hemihydrate gypsum and dissolved magraesi~~xm sulfate from the oxidation vessel, the alpharhemihydrate gypsum is separated 3D therefrom.

While alpha-hemihydrate gypsum is an especially useful form of gypsum and has specific uses for which conventional dehydrate gypsum, calcium sulfate dehydrate (CaS04~2Hz0), cannot be used, the production of conventional gypsum in efficient processes is also desirable.
It is an object of the present invention to provide a process of producing dehydrate gypsum, from an aqueous flue gas desulfurization slurry containing primarily calcium sulfite, in a manner that minimizes the air requirement for oxidation, reduces residence time in an oxidation vessel, and provides for oxidation of such slurries containing high solids concentration (25-35%).
It is another object of the present invention to provide a process of producing dehydrate gypsum, from an aqueous flue gas desulfurization slurry containing primarily calcium sulfite, using a pressurized oxidation vessel that effects gas bubble sizes and the mass transfer by providing a much higher surface area and thus better oxygen utilization, along with increased oxygen solubility.
SUMMARY OF THE INVENTION
The present process produces calcium sulfate dehydrate (dehydrate gypsum) from an aqueous flue gas desulfurization slurry containing primarily calcium sulfite. The aqueous slurry, containing calcium sulfite in a major amount by weight of the slurry solids, is charged, at a pH of about 5.8 to 6.0 into the top of a vertically oriented pressurized oxidation vessel and passed downwardly therethrough while contacting the aqueous slurry with a countercurrent flow of oxygen-containing gas. During the contacting, a pressure in the bottom portion of the oxidation vessel is maintained at about 22-32 pounds per square inch (psig), and a pressure in the top portion of the oxidation vessel is maintained at about 10-17 prig. Also, in the oxidation vessel, the aqueous slurry is maintained at a pH of about 4.0-5.5 and at a temperature of between about 180-217°F. Calcium sulfite in the aqueous slurry is oxidized to dehydrate gypsum and the slurry is removed from the bottom of the oxidation vessel at a pH of between 5.0-6Ø
BRIEF DESCRIPTION OF THE DRAWING
The drawing is a schematic illustration of the present process for producing dehydrate gypsum from an aqueous flue-gas desulfurization slurry containing primarily calcium sulfite.
DETAILED DESCRIPTION
The present process provides for the production of dehydrate gypsum from a flue gas desulfurization slurry containing primarily calcium sulfite by using a pressurized oxidation vessel under carefully controlled process conditions.
The process is for producing dehydrate gypsum from flue gas desulfurization slurries, such as calcium sulfite-containing slurries resulting from desulfurization systems using an aqueous slurry of lime or limestone as the reactant to remove sulfur dioxide, which reactants provide calcium sulfite by reaction with the sulfur dioxide. Such desulfurization systems produce an aqueous discharge slurry that contains primarily calcium sulfite as a major portion of the solids content of the slurry, and may contain a total, by weight, of about 10-35 percent solids.
The aqueous slurry, containing calcium sulfite is passed to the top portion of a vertically oriented pressurized oxidation vessel, with the pH of the aqueous slurry adjusted, if necessary to a pH of between about 5.8 to 6.0, preferably about 6.0, by addition of sulfuric acid prior to introduction into the pressurized oxidation vessel. The aqueous slurry passes downwardly through the vertically oriented pressurized oxidation vessel while an oxygen-containing gas is charged to the bottom portion of the pressurized oxidation vessel and passed upwardly, countercurrent to the aqueous slurry, the oxygen oxidizing the calcium sulfite to dehydrate gypsum.
The production of dehydrate gypsum in the pressurized oxidation vessel is provided by careful control of the conditions within the pressurized oxidation vessel. It has been found that the pressure at the bottom portion of the pressurized oxidation vessel must be maintained at between about 22 to 32 prig, preferably about 30 prig, while the pressure at the top portion thereof is maintained at between about 10-17 psig., most preferably between 13-17 prig. In addition, the temperature of the aqueous slurry in the pressurized oxidation vessel must be maintained between about 180-217°F, while the pH is maintained, by addition of sulfuric acid, at a pH of between about 4.0-5.5.
The calcium sulfite is oxidized, through contact with the oxygen of the oxygen-containing gas, to dehydrate gypsum under these controlled conditions, with the slurry containing resultant dehydrate gypsum discharged from the bottom portion of the pressurized oxidation vessel at a pH of between about 5.0-6Ø By use of the present process conditions, an oxidation efficiency of about 1.26:1 is achieved. That is, an air/sulfite stoichiometric ratio of about 1.26:1 is achieved, versus the earlier ratios of 2:1 to 3:1 known in the art, and referred to on page 2 hereof.
Referring now to the drawing, which schematically illustrates the present process, an aqueous flue gas desulfurization slurry containing solids that are primarily calcium sulfite is fed from a source, through line 1, to a supply unit 2.
Sulfuric acid is also fed to the supply unit 2 from source 3 through line 4, if needed, in an amount that will adjust the pH of the slurry to between about 5.8 to 6.0, preferably about 6Ø The aqueous flue gas desulfurization slurry, containing primarily calcium sulfite and at a pH of about 6.0, is then charged through line 5, by means of a higher pressure pump 6 into the top portion t of a vertically oriented pressurized oxidation vessel 7 which may contain a stirring mechanism 8 and wall baffles 9 which will aid in mixing of the slurry. Further sulfuric acid may be added to the pressurized oxidation vessel 7 through line 10 from a source 11.
The pressurized oxidation vessel 7 is a closed vertically oriented vessel which has a pressure release line 12 containing a pressure release valve 13. An oxygen-containing gas, preferably air, from a source 14 is charged to the bottom portion b of the pressurized oxidation vessel 7 through line 15 and spargers 16 so as to flow upwardly through the downwardly flowing aqueous slurry, i.e.
countercurrently. As shown by the dotted line, the aqueous slurry would have a volume V~ in the absence of air flow and a volume when air is charged to the pressurized oxidation vessel and flows countercurrently to the flow of aqueous slurry. The pressure at the bottom portion b of the pressurized oxidation vessel is maintained at about 22-32 psig, while the pressure at the top portion t of the pressurized oxidation vessel is maintained at a pressure of about 10-17 psig by regulation of the pressure release valve 13 on pressure release line 12 which release gases from the pressurized oxidation vessel 7. Also, the pH of the aqueous slurry is maintained at between about 4.0-5.5 by addition of sulfuric acid through line 10, and the temperature of the aqueous slurry maintained at between about 180-217°F. Under these conditions, the calcium sulfite in the aqueous slurry is oxidized to dehydrate gypsum, with the slurry containing the resultant dehydrate gypsum discharged from the bottom portion b of the pressurized oxidation vessel 7 through line 17 for collection at 18.
Example I
As an example of the present process, a sulfur dioxide flue gas desulfurization slurry containing calcium sulfite (23.9-31.9% solids) was charged to the top of a 40 foot long oxidation vessel having a diameter of 6 feet, at a pH of 5.0-5.5, and at a rate of 45-60 gallons/minute (GPM), and passed downwardly through the oxidation vessel, the slurry filling the reactor to a height of 26-28 feet with the height increased to 34-36 feet by injection of air, as an oxidizing gas, to the bottom of the oxidation vessel at a rate of 1475 standard cubic feet/minute (SCFM). The pressure in the oxidation vessel was maintained such that a pressure at the top thereof (vent pressure) was 13-15 psig, and the pressure at the bottom was 22-26 prig. The temperature was maintained between 190-205°F, and a pH of 4.0-5.0 maintained, with a mixer stirring the reactor slurry contents during flow downwardly therethrough.
The residence time, based on 28 feet of slurry was 1.64 hours at 60 GPM.
The oxidized slurry, containing calcium sulfate dehydrate (dehydrate gypsum), was discharged from the bottom of the vessel at a pH of 5.0-5.5, and contained uniform crystals of calcium sulfate dehydrate of a particle size of about 123 microns.
Example II
A further series of test runs were made using starting material as in Example I, under the conditions listed in Table I, with the results listed therein.

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As is seen from the above, the present process provides for the production of dehydrate gypsum in an efficient manner and high purity. Air or oxygen requirements are minimized, as are the residence time in the oxidation vessel. The process affects aas bubble sizes and the mass transfer by providing a higher surface area, along with increased oxygen solubility so as to effect efficient oxidation of calcium sulfite to dehydrate gypsum.

Claims (5)

1. A process of producing dehydrate gypsum from an aqueous flue gas desulfurization slurry containing primarily calcium sulfite, comprising:
charging the aqueous slurry containing calcium sulfite, at a pH of about 5.8 to 6.0, into a top portion of a vertically oriented pressurized oxidation vessel for downward passage therethrough;
contacting the aqueous slurry in the pressurized oxidation vessel with a countercurrent flow of an oxygen-containing gas;
maintaining a pressure in a bottom portion of the pressurized oxidization vessel at a pressure of about 22 to 32 psig and a pressure in the top portion of the pressurized oxidation vessel at a pressure of between about 10-17 psig, with the aqueous slurry maintained at a pH of about 4.0-5.5, and at a temperature of between 180-217°F, so as to convert calcium sulfite in the aqueous slurry to dehydrate gypsum; and discharging the slurry containing resultant dehydrate gypsum, at a pH of between about 5.0-6.0, from the bottom portion of the pressurized oxidation vessel.

2. The process of Claim 1 wherein said aqueous slurry containing calcium sulfite is charged to said pressurized oxidation vessel at a pH of about 6Ø

3. The process of Claim 1 wherein said pressure in the bottom portion of said pressurized oxidation vessel is maintained at about 30 psig.

4. The process of Claim 1 wherein said pressure in the top portion of said pressurized oxidation vessel is maintained at about 13-17 psig.

5. A process of producing dehydrate gypsum from an aqueous flue gas desulfurization slurry containing primarily calcium sulfite, comprising:
charging the aqueous slurry containing calcium sulfite, at a pH of about 6.0, into a top portion of a vertically oriented pressurized oxidation vessel for downward passage therethrough;
contacting the aqueous slurry in the pressurized oxidation vessel with a countercurrent flow of an oxygen-containing gas;
maintaining a pressure in a bottom portion of the pressurized oxidization vessel at a pressure of about 30 psig and a pressure in the top portion of the pressurized oxidation vessel at a pressure of between about 13-17 psig, with the aqueous slurry maintained at a pH of about 4.0-5.5, and at a temperature of between 180-217°F, so as to convert calcium sulfite in the aqueous slurry to dehydrate gypsum; and discharging the slurry containing resultant dehydrate gypsum, at a pH of between about 5.0-5.5, from the bottom portion of the pressurized oxidation vessel.