AU617665B2 - Process for converting calcium sulphate dihydrate into alpha-hemihydrate - Google Patents

Description

617665 COMMONWEALTH OF AUSTRALIA PATENTS ACT 1952 COMPLETE SPECIFICATION NAME ADDRESS OF APPLICANT: Saarbergwerke Aktiengesellschaft Trierer Strasse 1 D-6600 Saarbrucken Federal Republic of Germany NAME(S) OF INVENTOR(S): Jorg BOLD Frank FINK Jurgen UMLAUF ADDRESS FOR SERVICE: DAVIES COLJJSON Patent Attorneys 1 Little Collins Street, Melbourne, 3000.

COMPLETE SPECIFICATION FOR THE INVENTION ENTITLED: Process for converting calcium sulphate dihydrate into alpha-hemihydrate The following statement is a full description of this invention, including the best method of 'performing it known to me/us:rl 5 1 1 Description The invention regards a process for the conversion of calcium sulfate dihydrate from flue gas sulfur removal units into a bondable alphasemihydrate in a salt solution used as a calcinating agent, in which the wash liquid from the flue gas sulfur removal unit containing chloride is usea as a salt solution, and in which the obtained alpha-semihydrate is washed and dried.

A liquified calcium sulfate paste (gypsum paste) is ,obtained during wet flue gas cleaning with a wash liquid containing calcium ions. The gypsum can be separated from this paste in the form of a dihydrate.

With the gypsum dihydrate one obtains a product than can only be dissolved with difficulty in water, however, especially in the case of large firing installations, deposit problems arise due to the large amounts of gypsum produced.

For the economic utilization of gypsum from flue gas sulfur removal units it is known to convert gypsum dihydrate by proper thermal treatment into a bondable form, among others, as an alpha-semihydrate, which makes it possible to use the flue gas sulfur removal gypsum in the construction industry or in underground mining, such as, for example, for building walls and dams or to fill empty spaces.

2 For the crystallization conversion of dihydrate it is known to subject washed dihydrate largely free of chlorides to a hydrothermal treatment in a sulfuric acid solution or in autoclaves.

Further, from DE-OS 31 19 749 it is known to change the crystal structure of dihydrate from flue gas sulfur removal units without washing, that is, in the presence of a wash solution containing chloride which contains approx. 3 weight calcium chloride. The alpha-semihydrate obtained in this way is especially suited for its use in underground mining, since the sweating caused by the chloride contents are no problem there. Rather, the hardness characteristics are more important, which, according to DE-OS 31 19 749, are favorably influenced.

However, the conversion of dihydrate containing chloride cannot be implemented in a sulfuric acid solution since hydrochloric acid would be generated during contact with chloride. In the known process, the change of crystal structure is correspondingly implemented in the conventional manner inside autoclaves.

However, the conversion in autoclaves under pressure represents a large investment in machinery and energy and is, thus, very expensive.

Addationally, this type of installation requires a relatively large maintenance effort.

The task of the present invention consists in describing a simple ard economic process for the conversion of dihydrate stemming from flue gas 4.

4desulfurization plants.

The present invention provides process for the conversion of calcium sulfate dihydrate obtained from a flue gas desulfurization plant into alpha semihydrate by suspending the dihydrate in a salt solution used as a calcination agent comprising the following steps: mixing the calcium sulfate dihydrate with the salt solution; to obtain a slurry; heating the slurry; washing and drying the obtained alpha-semihydrate; characterised by using a scrubbing liquid from a flue gas desulfurization plant which has been concentrated to a concentration of calcium chloride of 20 to by weight as the salt solution; heating the slurry to a temperature of 85 to 98 *C until the conversion is at least substantially completed; washing the obtained a-semihydrate with a scrubbing liquid from a flue gas desulfurization plant which has not been substantially concentrated in order to reduce the salt content, and drying.

A preferred process may be characterised by further adding magnesium chloride or potassium chloride.

Preferably seed crystals are added to the slurry.

Preferably the seed crystals are added in an amount of 5 to 20% by weight based on the weight of the starting dihydrate.

Preferably the seed crystals are those obtained through the conversion of calcium sulfate dihydrate in a concentrated scrubbing liquid from a flue gas 910917,cnsdat.127,13991..r,3 size of 20 and 100 micrometers. This relatively large-grained product can be processed without any dust and is, therefore, especially suited for its use in mininq. Crystal formation my be accelerated by the addition of mra. nesium or potassium chloride.

If the crystal change takes place in a concentrated wash liquid with a calcium chloride contents above 45 weight and at temperatures above a fine grained semihydrate with crystal sizes 10 micrometers is cotained, which, due to the dust problems, is unsuited for underground mining. Since crystallization takes place faster within this range, seed crystals are aporopriately produced, and these are then added to the cih.drate. Preferably, these seed crystals are generated in a seed crystal reactor, into which one might also add the substances that influence the crystal structure or which accelerate the crystal formation.

Seed crystals can also be added by feeding back converted material.

The crystal structure of the obtained alpha-semihydrate aside from the mentioned addition of appropriate substances can also be influenced by the amount, size and shape of the added seed crystals. The amount of the adced seed crystals will appropriately be within the range of 5 to wei'&3ht percent in relation to the dihydrate.

If crystallization is implemented in several sequential stages following one another, the concentration of the substance influencing the crystal structure can be selected in a different manner within the inacvidual reaction stages, for example, in the seed crystal and in the

L,

gr'oth periods.

The obtained alpha-dihydrate is appropriately washed with wash water in several stages set up in a countercurrent from the flue gas sulfur removal unit so that, only that amount of calcium chloride is introduced into the process which is extracted with the unwashed dihydrate, which is then extracted together with the converted product and the concentration remains unchanged, that is, no foreign calcium chloride addition necessary. Appropriately, the wash water is preheated in an indirect heat exchange with the dried semihydrate and/or the vapors from the drier.

The process according to the invention is further explained by the process schematics shown as an implementation example in the figure.

Dihydrate, added by way of line 1, is mixed into a suspension with concentrated wash solution with a calcium chloride contents of 20 to weight coming in through line 9 from an intermediate tank 60. Because of the viscosity that increases with the dihydrate segment, the dihydrate segment of the suspension is limited to approx. 20 weight The substances that influence the crystal structure, or the substances that accelerate crystal formation may be added by way of line 2. The dihydrate suspension is transported into a reactor 20 by way of line 3 and, with the addition of heat, is crystallized there to alpha-semihydrate. In order to accelerate the conversion process seed crystals are added into reactor by.way of line lla, either in the form of fed-back converted material, or in the form cf seed crystals generated in seed reactor 80. The amount of a-Zsd seed crystals, in relation to the dihydrate, is of between 5 and wel..ht Preferably, part of the dihydrate suspension is extracted by way of lne 3 and is fed into an area of the reactor 20 by line 3a, in which a greater number of seed crystals with larger crystal surfaces is available.

H:-wever, in order to increase the solid contents, dihydrate may also be fed- back directly by way of line 14. This is possible because the volume of the generated alpha-semihydrate is smaller, and also, because in its crystalline form it is more compact and, thus more viscosity reducingthan the dihydrate.

The alpha-semihydrate generated in reactor 20 is extracted by way of lint 4, is washed, and the water is removed in a filtering unit 30 and is then fed into the drier 40 by way of line 5. The dried alpha-semihydrate is extracted from the unit by way of line 13.

Downstream from the filtering unit 30,by way of lines 11 and Ila, part of the converted product may be separated and may be reintroduced into the reactor 20 as seed crystals.

However, preferably, the seed crystals are generated in the seed reactor 80. Dihydrate is added to the seed reactor 80 by way of line la, or, possibly, by way of line 2a. Additives and concentrated wash solution i are added to the seed reactor 80 by lines 9 and 9a. Operation inside the seed reactor is appropriately accomplished with a wash solution with a calcium chloride contents of more than 45 weight at since at these temperatures crystallization accelerated manner.

temperatures above takes place in an In order to wash the alpha-semihydrate, wash solution from the flue gas sulfur removal unit is fed into the filtering unit 30 by way of line 17. The wash solution is appropriately preheated in heat exchangers arrd 90 by heat exchange with dried alpha-semihydrate and with the vapors from the drier. During this process, the calcium chloride attached to the crystal surfaces of the alpha-semihydrate is transferred to the wash solution. This ensures, that only the calcium chloride segment brought in with the unwashed dihydrate is extracted with the converted product.

Thus, one can omit using calcium chloride foreign to the processing unit.

The wash solution is extracted from the filtering unit by way of line 6 and is concentrated in a concentration vessel 50. Concentration is, for example, by thermal concentration. The generated condensed water is extracted through line 7. The wash solution thus concentrated is fed back into the intermediate tank 60 through line 8. A tank containing calcium chloride 70 only serves as a buffer used to adjust the concentration, or to compensate for the concentration fluctuations of the wash solution.

The process according to a separation of the calcium the wash solution. In this adequate for the reactor 20, the invention operates very economically if chloride has been provided by concentrating case, wasn solution with a concentration or for the seed reactor 80, may be extracted -at an appropriate location, and the wash liquid flowi'jng from washing and filtering unit 30 can.be fed backinto the water preparation stage of the flue gas sulfur removal unit.

Claims (7)

1. A process for the conversion of calcium sulfate dihydrate obtained from a flue gas desulfurization plant into alpha semihydrate by suspending the dihydrate in a salt solution used as a calcination agent comprising the following steps: mixing the calcium sulfate dihydrate with the salt solution; to obtain a slurry; heating the slurry, washing and drying the obtained alpha-semihydrate; characterised by I using a scrubbing liquid from a flue gas desulfurization plant which has been concentrated to a concentration of calcium chloride of 20 to by weight as the salt solution; heating the slurry to a temperature of 85 to 98 °C until the conversion is at least substantially completed; washing the obtained a-semihydrate with a scrubbing liquid from a flue gas desulfurization plant which has not been substantially concentrated in order to reduce the salt content, and drying.

2. A process according to claim 1, characterised by further adding magnesium chloride or potassium chloride.

3. A process according to claim 1 or claim 2 wherein seed crystals are added to the slurry.

4. A process according to claim 3, wherein the seed crystals are added in an amount of 5 to 20% by weight based on the weight of the starting dihydrate. A process according to claim 3 or claim 4 wherein the seed crystals are those obtained through the conversion of calcium sulfate dihydrate in a concentrated scrubbing liquid from a flue gas desulfurization plant with a calcium chloride content greater than 45% by weight at a temperature between 100 and 120 C. A process according to any one of claims 3 to 5, characterised by a 910917,cnsdat.127,13991res,1 I i i I-, S. S -11- continuous production and addition of the seed crystals, which are produced with an average residence time in a seed crystal producing reactor of 1 to minutes.

7. A process according to any preceding claim characterised by further adding 0 to 2% by weight of succinate, maleic acid or sulfonates to the slurry.

8. A process according to any preceding claim characterised by being performed as a continuous operation with an average residence time of the alpha-semihydrate in a reactor of 1 to 3 hours.

9. A process for the conversion of calcium sulfate dihydrate obtained fron a flue gas desulfurization plant substantially as hereinbefore described with reference to any one of the accompanying drawings. I Dated this 17th day of September, 1991. DAVIES COLLISON Patent Attorney for FRANK W. FINK, JORG BOLD and JURGEN UMLAUF 910917cmsdat.127,13991..res,2