DE3711367A1 - Process for preparing calcium sulphate hemihydrate - Google Patents

Abstract

The invention relates to a process for preparing calcium sulphate hemihydrate in the alpha configuration from an industrial waste material based on calcium sulphate dihydrate.

Description

The invention relates to a method for producing Calcium sulfate hemihydrate in alpha configuration from one industrial waste based on calcium sulfate dihydrate.

The dehydration of calcium sulfate dihydrate (CaSO₄ · 2H₂O, hereinafter called dihydrate) to calcium sulfate hemihydrate (CaSO₄ · 1 / 2H₂O, hereinafter called hemihydrate) is with a complete remodeling of the crystal lattice from monoclinic connected to rhombic symmetry. With dry dehydration Beta hemihydrate is formed during wet dehydration so-called alpha hemihydrate is formed.

The invention relates to the production of alpha hemihydrate from a synthetic starting material, in particular so-called Chemical and flue gas gypsum (the latter are also referred to as REA gypsum). These are the end product of flue gas desulfurization, for example in the wet washing process with limestone (CaCO₃) or with hydrated lime  (Ca (OH₂). Such a flue gas gypsum represents a calcium sulfate Dihydrate.

Such flue gas gypsum differ from natural gypsum in many ways. The flue gas gypsum is already in its raw state much more finely divided and shows through the manufacturing process Particle sizes of sometimes well below 50 µm. The crystal habit also differs from that of natural plaster from. In addition, there is the adsorptively bound surface moisture of the flue gas gypsum with about 10 wt .-% clearly above that of natural plaster (approx. 1 to 3% by weight).

Flue gas gypsum fall through the legally required Flue gas desulfurization in large quantities and there is an urgent need, this plaster of meaningful reuse feed.

The dehydration of the finely divided industrial plaster into alpha In the prior art, hemihydrate is such that initially Suspensions with a water content of over 50 wt .-% produced and then in an autoclave under the action of tense water vapor in water-saturated Environment to be drained.

This process requires a lot of equipment, because the suspensions in the autoclave during the conversion moved and subsequently with dewatering machines (for example Filter presses) must be dewatered before drying to remove the adsorptively bound residual water.

In addition, such methods are only used considerable amounts of energy feasible because not only the solids content (with a comparatively low  specific heat) heated to the reaction temperature must, but also the large amounts of water in the suspension.

Only a fraction of the energy used is therefore eliminated on the dehydration of the dihydrate.

This prior art is for example in DE-OS 31 14 363 describes a method of manufacture of alpha hemihydrate from a synthetic calcium sulfate Carrier claimed in which said aqueous slurry of calcium sulfate dihydrate with heating in one tubular reactor by introducing steam hydrothermally treated to form fibrous alpha hemihydrate becomes.

In addition, methods are also known in which the dehydration using crystallization catalysts is carried out, for example from DE-OS 33 31 838, but in turn from a dihydrate slurry is assumed.

In this context, DE-OS 35 33 007 is too call.

DE-OS 32 39 669 is a continuous process for the production of alpha hemihydrate by partial or complete hydrothermal conversion of industrial Waste gypsum in a tubular reactor using Steam at temperatures of 130 to 180 ° C known. Here is proposed to even out the crystal formation Superheated steam in cocurrent with the gypsum suspension once from above at certain flow velocities  down and then from the bottom up. Again, a plaster suspension with high Water content assumed what the disadvantages described conditionally.

In this respect, the invention is based on the object Process for the production of alpha hemihydrate in particular offer from industrial gypsum plaster, in which the apparatus and energy expenditure significantly reduced can be. With the invention, one should continue as possible complete conversion of dihydrate to alpha hemihydrate can be achieved. Finally, the aim is also that To design procedures so that it is preferably with existing Attachments can be executed.

The invention is based on the knowledge that it is not is necessary, an aqueous suspension as a starting material use which have a catalytic activity with regard the crystallization of alpha hemihydrate as a medium for the dehydration reaction. Rather, it has Invention recognized that a major problem in the Dehydration in the autoclave between the particles represents air. Through small and large air cushions between the individual solid particles of the starting material namely the heat transfer at subsequent autoclave treatment considerably more difficult. Depending on how the bed is made in the autoclave, comes irregularities occur even after prolonged autoclave treatment the temperature distribution in the autoclave and thus fluctuating degrees of dehydration and thus qualities of the end product.  

The invention proposes for the first time before actual autoclave treatment a ventilation of the autoclave or the material contained therein to make the air cushion between the solid particles to eliminate. A kind of "scaffold" remains from the solid particles with those in between Cavities in which the then in the autoclave fed steam then easily can penetrate easily and evenly.

Heating the starting material to the intended one Temperature takes place through the supply of heat, condensation of water vapor on the colder ones Surfaces of the solid particles is released.

The "open scaffolding" does not only make steam immediately facing surface very quickly the respective Accept temperature in the autoclave, but also the particles inside the bed to which the water vapor passes the vented voids of the fill quickly and directly can reach.

This will not only be a much faster, but even heat transfer to the goods is achieved, so that the dehydration taking place as well can run defined in a short time.

So it is crucial to achieve an "open porosity" within the "solid skeleton" of the bed.

Accordingly, the invention strikes in its most general Embodiment a method with the following steps:  

•  a) First, the waste material (for example a REA gypsum) placed in the autoclave. Depending on the size of the autoclave can do this, for example, by batching in different baskets or shapes are made, which are then placed in the autoclave. It can the material easily in the industrial Form can be used immediately.
• b) The autoclave is then vented. To this end, the invention proposes various alternatives in advantageous embodiments.
  One way of venting is to rinse the autoclave with steam. The water vapor pushes the air out between the individual particles of the material being fed in. The water vapor flushing can take place at atmospheric pressure.
  An alternative embodiment provides for the ventilation to be carried out by means of an evacuation device, for example a vacuum pump. The effect is the same, the air between the solid particles is extracted.
  In the third embodiment variant, the procedure is such that the autoclave is first charged with steam under the setting of a pressure of approximately 2 to 3 bar, possibly also higher. The air between the particles is compressed and does not initially escape. By subsequently relaxing (lowering the pressure) it is possible to pull out the air between the particles, which is removed at the same time. The greater the pressure drop (maximum to atmospheric pressure), the more vigorous the venting takes place. Experiments have shown that if a pressure of 2.5 bar is set in the first stage, a subsequent relaxation to approximately 1.5 bar is sufficient to achieve almost complete ventilation.
  Experiments have further shown that it is usually sufficient to carry out the evacuation up to about 70% in order to draw off so much air that sufficient pore space is subsequently available to lead the subsequently released tensioned water vapor completely and uniformly into the cavities can.
• c) As described above, the autoclave is then charged with steam under tension with a temperature in the autoclave between 105 and 180 ° C in a saturated steam atmosphere. The temperature is preferably 105 to 150 ° and in a more preferred range between 115 and 130 ° C.
  The saturation vapor pressure at a temperature of, for example, 130 ° C. is 2.8 bar.
• d) The moist atmosphere under pressure at the temperatures mentioned is maintained until the conversion of dihydrate into alpha hemihydrate has been carried out as completely as possible. A 100% conversion will generally not be possible. The term "complete conversion" is to be understood to mean the most complete conversion possible, for example 98% and more.
  In the case of an autoclave in which, for example, three baskets with 1 m³ of waste material each are placed, the holding time is approximately 1.5 hours.
•  e) The atmosphere in the autoclave is then brought to normal pressure relaxed.
•  f) The alpha hemihydrate formed can then be removed and according to level
•  g) then from adhering residual water (approx. 20-25% by weight from dehydration, condensed water and introduced moisture) can be freed by drying.

In contrast to the prior art methods can accordingly on separate drainage processes before Drying can be avoided as the material is a great deal has less moisture. Especially in manufacturing of alpha hemihydrate in large autoclaves and if the dryer capacity is limited the invention in an advantageous embodiment, the alpha hemihydrate formed after the pressure drop in Leave autoclaves at least partially at first. It so only the part that is immediately to be removed can then be tumble dried.

The remaining material is used to avoid any Rehydration during the time in the autoclave under a wet steam atmosphere at temperatures between 60 and 100 ° C, preferably kept at 90 ° C. The autoclave is used thus in this phase of the process as an interim storage facility. In the prior art, however, an intermediate storage was required always take place in heated silos. This is also an issue Another advantage of the method according to the invention.  

To avoid rehydration, the final one Drying, for example in flow tube dryers, drying drums, heated screws, cyclones or the like can be made at temperatures above 80 ° C, preferably between 80 and 150 ° C. Preferably the drying temperature is between 100 and 130 ° C.

The invention also proposes further energy savings before, the dryer waste heat in the autoclave after Pressure drop according to stage e). Naturally can also be live steam or other at this point Waste steam or hot air can be supplied.

Further features of the invention result from the others Claims and the other application documents.

The invention is described below using various exemplary embodiments explained in more detail.

For the practical application of a plaster are on the one hand the properties of the set gypsum body such as strength, Adhesiveness and density are decisive, on the other but also the behavior of the gypsum paste during solidification and in this context the amount of litter, bending tension and compressive strength.

Flue gas desulfurization gypsum was used in the tests of different origins with moisture contents between 6 and 23 wt .-% and a proportion of calcium sulfate dihydrate of over 90% by weight and a fineness of less than 100 µm used.  

In the test series A, the process was without venting of the autoclave, in test series B with ventilation the autoclave by steam rinsing. In both cases the autoclave treatment was carried out via 90 minutes at 130 ° C (corresponding to approximately 2.8 bar).

The following values resulted:

Test series A

Test series B

A comparison of the results of the test series A with those Test series B shows that those with the invention Processes produced alpha hemihydrate approximately  to double the tensile and compressive strengths to lead. Among other things, this is based on a particularly homogeneous, almost exclusively rod-shaped, compact crystal form of the alpha hemihydrate. In contrast to show the samples treated after the test series A. a very different, acicular, fibrous crystal habit.

As described, the method according to the invention can also be used conventional autoclaves. Continue it should be noted that the invention Of course, this procedure also applies to dehydration natural gypsum to be applied to alpha hemihydrate can, although there the problems described at the beginning not available to the same extent according to the state of the art are because in the case of natural gypsum in the form of "gypsum cartilage", that is, large, compact pieces abandoned where the individual pieces are large-volume between themselves Form pores over which when heating the Autoclaves by introducing steam more easily a more even Temperature transfer is possible.

The invention also relates to a mortar mixture based on one as described above Process prepared calcium sulfate hemihydrate the alpha configuration and one or more residues from a spray adsorption desulfurization process as well if necessary, other additives and additives.

So this is a mortar mixture the basis of two residues from desulfurization processes, which advantageously combines the Production of an advantageous mortar, in particular  for use underground, as a so-called mining Mortar can be used.

There is an urgent need, especially in mining for mortars that should be inexpensive and in large Amounts are needed. Known cement grades divide because of their relatively high price due to the large Share in Portland cement clinker. For mining use, for example as backfill material the mortars and concretes are not strong, which, for example, reach cements. Rather is due the sometimes long transport routes require that the corresponding Mortar or concrete an adjustable solidification and have hardening behavior to a light and to ensure safe processability.

The mortar mixture mentioned can meet these requirements, which, moreover, for the dismantling of the considerable quantities at Flue gas desulphurization contributes residues.

The residues from the spray adsorption desulfurization process in a grade between 10 and 60% by weight, preferably 30 to 50% by weight, based on the total mixture.

The residue, which is also part of the incineration process resulting fly ash may exist all of calcium sulfate dihydrate and / or calcium sulfate Hemihydrate and / or calcium sulfate and / or calcium sulfite Hemihydrate. For a preferred mortar mix, before all calcium sulfite hemihydrate used.

The fly ash that may be carried in the residues Share can by separate fly ash, so another  Residual product to be supplemented. The fly ash also serves as a hydraulic exciter and improves the strength development and property of the mortar additionally.

It is also particularly preferred to add a coarse grain, so-called sweeping grain, for example limestone chippings or Gravel, preferably with a grain size of 1 to 3 mm, for pneumatic Transport deposits in the conveyor ways and caking in the area of the To prevent discharge.

Finally, other additives, such as plasticizers or retarders, for example surfactants, lignin sulfonates or the like are added. Other characteristics result itself from the other claims.

Claims (21)

1. A process for producing calcium sulfate hemihydrate in alpha configuration from an industrial waste material based on calcium sulfate dihydrate, with the following steps:

• a) Feeding the waste material into an autoclave
• b) Bleeding the autoclave
• c) Charging the autoclave with tensioned steam while setting a temperature in the autoclave between 105 and 180 ° C.
• d) maintaining the hydrothermal conditions according to c) until preferably complete conversion of calcium sulfate dihydrate into calcium sulfate hemihydrate of the alpha configuration (so-called alpha hemihydrate)
• e) Pressure reduction in the autoclave to normal pressure
• f) removal of the alpha hemihydrate formed
• g) Freeing the alpha hemihydrate from adhering residual water by drying.

2. The method according to claim 1, wherein for venting the Autoclave according to step b) with steam is rinsed. 3. The method of claim 2, wherein the rinsing with steam at atmospheric pressure. 4. The method of claim 1, wherein for venting the Autoclave according to step b) this is evacuated. 5. The method according to claim 4, wherein the development means a vacuum pump. 6. The method according to claim 1, wherein the venting of the autoclave according to step b) is carried out by the following, briefly successive substeps:

• b1) loading the autoclave with steam while setting a pressure of approximately 2 to 3 bar,
• b2) Lowering the pressure by at least 0.9 bar to approximately 1.1 to 1.8 bar.

7. The method according to any one of claims 1 to 6, wherein the Temperature according to step c) between 105 and 150 ° C, preferably 115 to 130 ° C is set. 8. The method according to any one of claims 1 to 7 with the Provided that a calcium sulfate dihydrate carrier as waste from the flue gas desulfurization is used (so-called REA gypsum).   9. The method according to any one of claims 1 to 8, wherein the waste material before autoclaving mineralizers, especially polycarboxylic acid, especially amber, Malic, maleic, citric, methyl succinic acid or their salts and / or inorganic sulfates, in particular Potassium sulfate and aluminum sulfate can be added. 10. The method according to any one of claims 1 to 9, wherein the alpha hemihydrate formed before removal the autoclave according to step f) during a certain Time in the autoclave while maintaining one Wet steam atmosphere at 60 to 100 ° C, preferably 90 ° C remains. 11. The method of claim 10, wherein the wet steam atmosphere by supplying fresh or waste steam and / or from dryer waste air or fresh hot air becomes. 12. The method according to any one of claims 1 to 11, wherein drying according to step g) at one temperature between 80 to 150 ° C, preferably between 100 and 130 ° C. 13. Calcium sulfate hemihydrate in alpha configuration with rod-shaped, compact crystal shape according to the method of one or more of the claims 1 to 12. 14. Mix mortar with

• 1. calcium sulfate hemihydrate of the alpha configuration according to claim 13,
• 2. one or more residues from a spray adsorption desulfurization process and optionally
• 3. other additives and additives.

15. Mortar mixture according to claim 14, wherein the residues from the spray adsorption desulfurization process in a content between 10 and 60 wt .-%, preferably 30 to 50% by weight, based on the total mixture, are available. 16. Mortar mixture according to claim 14 or 15, wherein the Residues predominantly from calcium sulfite Hemihydrate exist. 17. mortar mixture according to one of claims 14 to 16, where the calcium sulfate hemihydrate content of the alpha Configuration 40 to 90% by weight, preferably 50 to 70% by weight, based on the total mixture. 18. Mortar mixture according to one of claims 14 to 17 with the addition of a sweeping grain, preferably a limestone chippings. 19. Mortar mixture according to claim 18, wherein the sweeping grain with a grain size between 1 and 3 mm. 20. Mortar mixture according to one of claims 14 to 19 with the addition of fly ash. 21. Mortar mixture according to one of claims 14 to 20 with the addition of a solidification of the mortar mixture regulating and / or liquefying after adding water Additive, preferably in an amount between 0.1 and 1 wt .-%, based on the total mixture.