DE3331838A1 - Process for converting the residue from flue gas desulphurisation plants to crystalline gypsum alpha-hemihydrate - Google Patents

Abstract

In a process for converting the residue, mainly containing gypsum dihydrate, from flue gas desulphurisation plants, in particular from wet flue gas desulphurisation plants, to crystalline gypsum alpha-hemihydrate by dehydration with sulphuric acid at a preset reaction temperature, a dihydrate suspension is formed from the residue with sulphuric acid and, if appropriate, water. The dihydrate suspension is heated in a reaction vessel and held at reaction temperature until a sufficient conversion to crystalline gypsum alpha-hemihydrate has taken place. A suspension of alpha-hemihydrate is taken off from the reaction vessel. A portion of the suspension of alpha-hemihydrate remains in the reaction vessel as a starting suspension for the conversion of further gypsum dihydrate. The starting suspension is mixed with a further reaction quantity of a dihydrate suspension made up with sulphuric acid and, if appropiate, water, and the mixture is again held in the reaction vessel at reaction temperature until a sufficient conversion to crystalline gypsum alpha-hemihydrate has in turn taken place and a product suspension has thus been formed. A part of the product suspension is then taken off and taken to further processing, while the remainder remains in the reaction vessel, and so on, with admixture of further reaction quantities to the part quantity, which has remained in the reaction vessel, or remaining part quantity of the product suspension.

Description

Alterjewski, Honke & Partner, patent attorneys in Essen

The invention relates to a method for converting the mainly dihydrate gypsum containing gypsum Residue from flue gas desulfurization systems, especially from wet flue gas desulfurization systems, in alpha hemihydrate gypsum crystals by dehydration with sulfuric acid at a given reaction temperature,

whereby a dihydrate slurry is formed from the residue with sulfuric acid and optionally water, which Dihydrate slurry heated in a reaction vessel and is kept at the reaction temperature until sufficient conversion to alpha hemihydrate gypsum crystals has taken place,

and wherein from the reaction vessel an alpha hemihydrate slurry is deducted, the z. B. can be further processed to building plaster.

It goes without saying that the residue from flue gas desulfurization systems in addition to dihydrate gypsum, albeit in small amounts, other ingredients, mainly calcium sulfite and . Includes flying bag. For this reason, the residue from flue gas desulphurisation systems is converted into alpha hemihydrate gypsum crystals a particularly difficult problem and with other measures for the production of alpha hemihydrate gypsum hardly comparable. In the context of the invention, dihydrate denotes slurry for the purpose of differentiating resulting from such a slurry by reaction Slurries the slurry of the whole, only residue containing mainly dihydrate gypsum

Andrejewski, Honke & Partner, patent attorneys in Essen

from flue gas desulphurisation systems. Alpha hemihydrate slurry means a slurry which mainly contains apha hemihydrate gypsum crystals, but also dihydrate gypsum crystals and other components from the residue of the flue gas desulfurization with it. It goes without saying that Intensive mixing, mostly by stirring, takes place in the reaction vessel in addition to the heating.

In the known generic method (DE-OS 31 29 878, page. 3, line 1 to page 4, line 3) from the Alpha hemihydrate slurry formed dihydrate slurry withdrawn as product from the reaction vessel. A new dihydrate slurry is then made up and as described treated, mostly at atmospheric pressure and around the boiling point. The resulting alpha hemihydrate gypsum has neither sufficient nor consistent quality. An improvement due to defined thermodynamic conditions brings a continuous procedure (DE-OS 31 29 878, page 4, line 4 to page 6), in which the residue in the form of desulphurisation sludge as a layer cake on a vapor-permeable Pad applied and by supplied sulfuric acid and / or supplied steam in the layer cake becoming dehydrated. A vacuum belt filter is used for this.

The invention is based on the object of carrying out the generic method using a reaction vessel in such a way that that alpha hemihydrate gypsum crystals of high quality can be produced reproducibly.

Alterjewski, Honke & Partner, patent attorneys in Essen

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To solve this problem, the invention teaches / that a portion of the alpha hemihydrate slurry as a starting slurry for the conversion of further dihydrate gypsum is left in the reaction vessel,

that the starting slurry with a further amount of reaction mixed with a dihydrate slurry made up with sulfuric acid and optionally water and the The mixture in the reaction vessel is kept at the reaction temperature until there is sufficient reaction again takes place to alpha hemihydrate gypsum crystals and thereby a product slurry is formed,

that then a portion of this product slurry is withdrawn and fed to further processing, while the remainder remains in the reaction vessel - and so on with admixture of further conversion amounts to that in the reaction vessel 'remaining portion of the product slurry. To A preferred embodiment of the invention is, as also within the framework of the known measures, with atmospheric pressure and worked in the range of the boiling point of the slurry. However, it is within the scope of the invention to carry out the reaction in an autoclave at elevated pressure and elevated temperature perform. In any case, it is advisable to convert the residue, like the other amounts of reaction, into a dihydrate slurry to make the liquid content of a 7.5 to 30%, preferably a 10 to 25% sulfuric acid is equivalent to. You get particularly good results when you work with a liquid fraction, the one

Andrejewski, Honke & Patiner, patent attorneys in Essen

Corresponds to 20% sulfuric acid. By choosing the acid concentration the reaction time can be set, which is essential for the treatment of the mixture in the reaction vessel. This is explained further below with the aid of graphic representations. It is best to keep track of the residue, as well such as the further conversion quantities to make a dihydrate slurry, its sulfuric acid / gypsum ratio in the range from 0.4 to 2, preferably about 0.6, and this ratio must also be maintained in the reaction vessel. The range from 0.6 to 1.0 has also proven its worth. - In the context of the process according to the invention, the residue can dry or wet, as z. B. from a wet desulphurisation plant is used, in the latter case the water content when setting the acid concentration is to be considered. Preferably, to adjust the acid concentration with fresher, virgin Sulfuric acid worked. However, it is within the scope of the invention for the further processing of the Sulfuric acid filtrate with the product slurry concentrate fresh, concentrated sulfuric acid again to preferably 20% by weight sulfuric acid and the Re-use acid for dehydration.

The invention is based on the knowledge that alpha hemihydrate gypsum crystals Surprisingly high quality produced reproducibly from the residue from desulphurisation plants if this alpha hemihydrate gypsum is given the opportunity to grow on crystal nuclei that in turn are formed from the residue and are available in sufficient numbers. Therefore, the invention works in the described manner with a special starting slurry

Alterjewski, Honke & Partner, patent attorneys in Essen

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and the invention leaves a portion of the product slurry, as a further starting slurry, in the reaction vessel In this respect, autogenous crystal nuclei are used as it were worked, which are formed from the residue of the flue gas desulfurization itself and which surprisingly turns out to be have proven to be much more effective than foreign crystal nuclei or other crystallization aids like them are commonly used in the production of alpha hemihydrate gypsum. According to a preferred embodiment of the invention is a sufficient number of such crystallization nuclei in the starting slurry or in the remainder of the product slurry remaining in the reaction vessel and thus contained in the newly reacted mixture when the alpha hemihydrate / dihydrate ratio is measured in the slurry (by drawing and sampling from the reaction vessel) and with an alpha hemihydrate / dihydrate ratio of 50: 1 or less, the alpha hemihydrate slurry or slurry Product slurry is withdrawn. This measurement is can be easily carried out by determining the water of crystallization of the mixture of alpha hemihydrate / dihydrate crystals. Man but can also connect to the acid concentration in order to control the process according to the invention, because in the Conversion of dihydrate gypsum to alpha hemihydrate gypsum crystals releases crystal water and consequently the acid concentration decreases. In this context, the invention teaches that in the slurry in the reaction vessel the sulfuric acid concentration, measured and at an acid concentration that is less than that of the by a predetermined amount Dihydrate slurry, the alpha hemihydrate slurry or the product slurry is withdrawn. Is working within the scope of the invention with constant parameters with regard to the residue from the flue gas desulphurisation, if the flue gas desulphurization is run stationary,

Andrejewski, Honke & Partner, patent attorneys in Essen

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the method according to the invention can thus be controlled purely in terms of time as will be explained on the basis of the exemplary embodiments. In any case, the further conversion quantities can iteratingly mixed into the reaction vessel in portions or are introduced continuously into the reaction vessel.

The advantages achieved are to be seen in the fact that, according to the invention, from the residue mainly containing dihydrate gypsum High quality alpha hemihydrate gypsum crystals produced in a reproducible manner from flue gas desulphurisation systems can be. This is particularly true when the residue has a fly ash content of 0.2 to 0.8% by weight. Residues of dihydrate gypsum carried along in the product slurry are harmless for many applications, but can also be eliminated by a suitable dehydrating post-treatment.

In the following the invention is based on a schematic Representation explained further. Show it:

Fig. 1 shows a reaction vessel in which the inventive method is carried out,

2 shows a graphical representation showing the relationships in the case of a conversion of dihydrate gypsum to alpha hemihydrate gypsum crystals basically explained and

Alterjewski, Honke & Partner, patent attorneys in Essen

3 shows a graphic representation corresponding to FIG. 2, which relates to the conversion ratios in the case of the invention Procedure is specified.

The residue from flue gas desulfurization systems, in particular, which contains mainly dihydrate gypsum, is converted from wet flue gas desulphurisation systems. This residue is first made into one with sulfuric acid and, if necessary, water Dihydrate slurry in the sense of the introductory definition educated. This dihydrate slurry is introduced into the reaction vessel 1 shown in FIG. It is a reaction vessel with a stirrer 3 which can be heated by means of a double jacket 2 and steam in the exemplary embodiment can be carried out at atmospheric pressure and in the vicinity of the boiling temperature. The following will first of all the iterative procedure already explained is described:

The dihydrate slurry is heated in the reaction vessel and kept at the reaction temperature until sufficient conversion to alpha hemihydrate gypsum crystals is done. If this implementation has taken place, the slurry now mainly containing alpha hemihydrate gypsum crystals, namely the alpha hemihydrate slurry, deducted. However, a portion of the alpha hemihydrate slurry remains as the starting slurry in the reaction vessel 1, as indicated by the dashed line 4. It is used as a start slurry for the implementation of further Leave the dihydrate gypsum in the reaction vessel 1. The start-up

Andrejewski, Honke & Partner, patent attorneys in Essen

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_ τ 2 - Zf ^ f \ j S W ^ - U

However, slurry will involve a further amount of conversion a dihydrate slurry made up with sulfuric acid and possibly water mixed. This is supplied at 5. Thereafter, the mixture in the reaction vessel 1 is heated to the reaction temperature again held until sufficient conversion to alpha hemihydrate gypsum crystals has taken place and thereby the actual product slurry is formed. Part of this product slurry is withdrawn at 6 and fed to further processing, while again a remainder, roughly corresponding to line 4 in reaction vessel 1 remains. This is followed by an admixture of further amounts of conversion to this partial amount remaining in the reaction vessel the product slurry - and so on. The amount of product slurry left in the reaction vessel and thus the further conversion quantities can be easily optimized. A ratio of about 2 5: 7 5 has proven to be useful always about 25% of the alpha hemihydrate slurry deducted and replaced by further turnover quantities.

In FIG. 2 one recognizes a graphic representation, on the ordinate of which the water of crystallization of gypsum crystals can be seen is plotted, while the time was plotted on the abscissa. This graphic representation describes with the solid curve shows the relationships in the known procedure from which the invention is based. One recognises, that the dihydrate slurry used has gypsum crystals, whose water of crystallization content is 20% by weight. A reaction occurs as a result of the treatment in the reaction vessel.

Andrejewski, Honke & Partner, patent attorneys in Essen

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The dihydrate gypsum changes increasingly over time into alpha hemihydrate gypsum crystals. Alpha hemihydrate gypsum crystals have a water of crystallization content of 6% by weight. The implementation can be traced by the fact that the in the mixture of dihydrate gypsum and gypsum contained in the slurry;] Alpha hemihydrate gypsum crystals lose crystal water. The curve shown shows you more or less horizontally running area with a water of crystallization content j

i of about 6% by weight. This area corresponds to the practically complete implementation> in the case of alpha hemihydrate gypsum crystals, predominate in the slurry. Depending on the sulfuric acid concentration of the proportion of liquid in the slurry, the curve is more or less in the sloping part steep and the horizontal branch of the curve, determined by the alpha hemihydrate gypsum crystals, is more or less pronounced. This is indicated by the dash-dotted curves drawn in FIG. 2. The unsatisfactory conditions at the known procedure are based on the fact that one with Insufficiently complete conversion of the dihydrate gypsum works for alpha hemihydrate gypsum crystals and / or accepts that the water of crystallization is higher than that for alpha hemihydrate gypsum crystals typical water of crystallization content of about 6 wt .-% is further degraded.

The situation is completely different with the one according to the invention Procedure that was explained in FIG. 3. Here, too, the water of crystallization is on the ordinate Plaster of paris crystals applied in the slurry in the reaction vessel, time on the abscissa. One recognizes after that

Andrejewski, Honke & Partner, patent attorneys in Essen

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horizontal curve branch sawtooth-like curve formations, which repeat themselves after certain time intervals ^ t. The steep flank of the sawtooth-like curve formations indicates that another amount of conversion has been added here (after a portion of the alpha hemihydrate slurry was previously added withdrawn as a product slurry). By adding the new conversion amount has the proportion of dihydrate gypsum and thus the value of the water of crystallization in the Mixture of the slurry increased again, only to drop again down to that for alpha hemihydrate gypsum crystals typical value - and so on.

3 describes the method of operation in which iterating with, as it were, portion-wise withdrawal of the product slurry and the addition of further amounts of conversion is carried out in portions. One recognizes that in this respect with otherwise the same Process parameters can also be controlled purely in time, for example with a ^ t of could work about 6 minutes.

In addition, the expert inferred from this graphical representation, that also a continuous mode of operation with continuous withdrawal of product slurry and continuous The addition of further amounts of conversion is possible if it is only ensured that there are in the reaction vessel an average residence time for the individual volume elements of the slurry that corresponds to the teachings of the invention.

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Claims (10)

Alterfewski, Honke & Partner patent attorneys Physicist Dr. Walter Andrejewski Graduate engineer Dr.-Ing. Manfred Honke 331838 Graduated physicist Dr. Karl Gerhard Masch Lawyer files: 4300 Essen 1, THeaterplatz 3, Postf. 10 02 60 578 / Ma- August 18, 1983 Patent application Gottfried Bischoff construction compl. Gas cleaning and water recooling systems GmbH & Co. Limited partnership Gärtnerstrasse 44 4300 Essen 1 "Process for converting the residue from flue gas desulphurization systems into alpha hemihydrate gypsum crystals" Patent claims: ι I _x I process for converting the residue, mainly containing dihydrate gypsum, from flue gas desulfurization systems, in particular from wet flue gas desulfurization systems, to form alpha hemihydrate gypsum crystals by dehydration with sulfuric acid at a given reaction temperature, whereby from the residue with sulfuric acid and possibly Water formed a dihydrate slurry and the dihydrate slurry in a reaction vessel. copvJ ι. . Andrejewski, Honke & Partner, patent attorneys in Essen warms and is kept at reaction temperature until sufficient conversion to alpha hemihydrate gypsum crystals has taken place, and wherein from the reaction vessel an alpha hemihydrate slurry is deducted, the z. B. can be further processed to building plaster, characterized in that that a portion of the alpha hemihydrate slurry as a starting slurry for the implementation of further dihydrate gypsum is left in the reaction vessel, that the starting slurry with a further conversion amount mixed with a dihydrate slurry made up with sulfuric acid and optionally water and the The mixture in the reaction vessel is kept at the reaction temperature until there is sufficient reaction again to alpha hemihydrate gypsum crystals and thereby formed a product slurry is, that then a portion of this product slurry is withdrawn and fed to further processing, while the remainder is left in the reaction vessel - and so on with admixture of further conversion amounts to the portion of the product slurry remaining in the reaction vessel. 2. The method according to claim 1, characterized in that the reaction is carried out at atmospheric pressure and around the boiling point of the slurry. Andrejewski, Honke & Partner, patent attorneys in Essen 3. The method according to any one of claims 1 or 2, characterized in that that the residue, as well as the further conversion quantities, made up to a dihydrate slurry whose liquid content corresponds to a 7.5 to 30% strength, preferably a 10 to 25% strength sulfuric acid. 4. The method according to claim 3, characterized in that it is carried out with a liquid portion, the one Corresponds to 20% sulfuric acid. 5. The method according to any one of claims 3 or 4, characterized in that that the residue, as well as the further conversion quantities, are made into a dihydrate slurry, their sulfuric acid / gypsum ratio in. The range is from 0.4 to 2, preferably about 0.6, and that this Ratio in the reaction vessel is maintained. 6. The method according to any one of claims 1 to 5, characterized in that that the alpha hemihydrate / dihydrate ratio is measured in the slurry in the reaction vessel and with an alpha hemihydrate / dihydrate ratio of at most 50: 1, the alpha hemihydrate slurry or the Product slurry is withdrawn. 7. The method according to any one of claims 1 to 5, characterized in that that in the slurry in the reaction vessel the sulfuric acid concentration is measured and at an acid concentration by a predetermined amount is smaller than that of the dihydrate slurry, the alpha Andrejewski, Honke & Partner, patent attorneys in Essen semi-hydrate slurry or the product slurry is deducted. 8. The method according to any one of claims 1 to 7, characterized in that that the further conversion amounts are mixed in portions into the reaction vessel. 9. The method according to any one of claims 1 to 7, characterized in that that the other amounts of reaction are continuously mixed into the reaction vessel. 10. The method according to any one of claims 1 to 9, characterized in, that a residue is used which has a content of 0.2 to 0.8% by weight of fly ash.