FR2613346A1 - PROCESS FOR PRODUCING CALCIUM SULFATE SEMI-HYDRATE, CALCIUM SULFATE SEMI-HYDRATE PRODUCED BY CARRYING OUT SAID PROCESS AND MIXTURE OF MORTAR CONTAINING SUCH SEMI-HYDRATE - Google Patents

Abstract

THE PRESENT INVENTION CONCERNS A PROCESS FOR THE MANUFACTURING OF CALCIUM SULPHATE SEMI-HYDRATE IN ALPHA CONFIGURATION FROM AN INDUSTRIAL RESIDUE BASED ON CALCIUM SULPHATE DIHYDRATE, CONSISTING OF CHANGING THE MATERIAL IN AN AUTOCAERLAVE, A DES , TO SUPPLY IT WITH PRESSURIZED WATER VAPOR, AT A TEMPERATURE OF 105 TO 180 C THEN, AFTER TRANSFORMATION OF THE DIHYDRATE, TO RETURN TO NORMAL PRESSURE, TO REMOVE THE SEMI-HYDRATE ALPHA, AND TO ELIMINATE BY DRYING THE RESIDUAL WATER ADHERED TO THIS. THE INVENTION CONCERNS CHEMICAL PLASTERS AND FLUE GAS DESULFURATION PLASTERS AND IT IS MORE PARTICULARLY AIMED AT THE MANUFACTURE OF SEMI-HYDRATE ALPHA FROM INDUSTRIAL REBUS PLASTERS.

Description

Process for the production of calcium sulphate hemihydrate

  The present invention relates to a process for the manufacture of calcium sulphate hemihydrate in alpha configuration from a residue

  based on calcium sulfate dihydrate.

  The dehydration of calcium sulfate dihydrate (CaSO 4 · 2H 2 O, hereinafter referred to as "dihydrate") to calcium sulphate hemihydrate (CaSO 4 '1 / 2H 2 O, hereinafter referred to as "hemihydrate") is accompanied by a total transformation of the crystal lattice, which passes from monoclinic symmetry to orthorhombic symmetry. With dry dehydration, semi-hydrate beta is obtained, while wet dehydration provides

alpha hemihydrate.

  The present invention is concerned with the manufacture of alpha hemihydrate from an artificial starting material, particularly the so-called chemical plasters and flue gas desulfurization plasters. These plasters are the final product of flue gas desulfurization, for example by a wet scrubbing process with limestone (CaCO3) or with hydrate of lime (Ca (OH2)). Such a plaster of gas desulfurization

  of smoke is a calcium sulfate dihydrate.

  Such plasters are different from natural plasters in many aspects. In the raw state, the flue gas desulphurization plaster is already much thinner and, due to the manufacturing process, has a particle size that is in part much less than 55 μm. The characteristic shape of the crystals differs from that of natural plasters. In addition, the adsorbed surface moisture of the flue gas desulphurization plaster is, with about 10% by weight, substantially greater than that of the flue gas desulfurization plaster.

  natural plasters (about 1 to 3% by weight).

  Plasmas of desulphurisation fall under the legal provisions concerning the desulphurisation of flue gases and there is therefore a

  urgent need to provide them with valuable reuse.

  According to the state of the art, the dehydration of industrial plasters

  alpha hemihydrate is carried out by first preparing

  pensions having a water content in excess of 505% by weight, then dehydrating these suspensions in an autoclave under the action of steam

  under pressure, in an atmosphere saturated with water.

  This method requires a cost of equipment élex é. because the suspensions move during their transformation in the autoclave and must then be dehydrated using dehydration machines (filter presses for example), before drying them to remove the residual water bound by adsorption. In addition, such processes require considerable amounts of energy because it is necessary to heat to the reaction temperature not only the share of solids (whose specific heat is relatively low), but also the large amounts of water present in suspension. As a result, only a fraction of the energy consumed is used for

dehydration of the dihydrate.

  This state of the art is for example described in the German patent application DE-31 14 363, which discloses a process for the manufacture of alpha hemihydrate from an artificial calcium sulfate support, according to which the aqueous suspension of calcium sulphate dihydrate is hydrothermally treated by reheating in a tubular reactor with steam introduction to form alpha half-hydrate.

fibrous.

  Also known are processes according to which the dehydration is carried out employing crystallization catalysts, as for example in the German patent application DE-33 31 838; however, this document

  part again of a suspension of dihydrate.

  In this context, the patent application must also be cited.

German DE-35 33 007.

  DE-32 39 669 discloses a continuous process for the production of alpha hemihydrate by partial or total hydrothermal conversion of industrial waste plasters in a tubular reactor using steam at temperatures ranging from 130 to 180 C. It is proposed, in order to standardize the crystallization, to guide the hot steam flowing in the same direction as the plaster suspension, at given flow rates, from top to bottom then to bottom up. Again, here we start with a plaster suspension with a high water content, which

  causes the aforementioned drawbacks.

  The object of the present invention is to propose a process for the manufacture of alpha hemihydrate, in particular from industrial waste plasters, with which the cost of equipment and energy can be significantly reduced. The invention should further provide a transformation if possible total dihydrate alpha hemihydrate. Finally, we try to configure the process so that it can be preferably

  existing installations.

  The invention is based on the finding that it is not necessary to employ an aqueous suspension as a starting material having a catalytic activity for the crystallization of alpha hemihydrate as an agent for the dehydration reaction. On the contrary, it has been found that a major problem encountered during the dehydration in the autoclave was due to the air present between the particles. Indeed, the presence of air cushions of greater or lesser size between the different particles of solid material of the starting material considerably hinders the heat transfer during the subsequent treatment in an autoclave. Depending on the manner in which the feed is discharged into the autoclave, even with long-term autoclave treatment, irregularities in the temperature distribution in the autoclave and subsequent

  the degree of dehydration and the quality of the final product.

  The invention proposes for the first time to undertake, before the actual autoclave treatment, a deaeration of the autoclave or of the material which is in the latter, in order to eliminate the air cushions present between the particles of solid material. In doing so, a kind of "framework" is formed consisting of the particles of solid material between which are cavities, in which the pressurized water vapor which is then fed into the autoclave can penetrate easily and uniformly. The heating of the starting material at the expected temperature takes place here by the contribution of the heat which is released during the condensation of the

  water vapor on colder surfaces of solid particles.

  Thanks to the "open frame", it is not only the surface directly turned to the steam that will quickly take the temperature present in the autoclave, but also the particles that are inside the load, that the steam of water can reach quickly and

  directly through the deaerated cavities of the load.

  This gives a heat transfer on the product which is not only much faster but also more uniform, so that the dehydration which is accomplished then can take place in a short time and

also defined way.

  The decisive element is thus to obtain an "open porosity" to

  inside the "skeleton of solid material" loading.

  Consequently. the invention proposes in its most general embodiment a process comprising the following steps: a) Firstly, the residual material (flue gas desulfurization plaster for example) is loaded into the autoclave. Depending on the size of the autoclave, this can be done by feeds fed into different baskets or bags which are then placed in the autoclave. In doing so, the material can be used directly in its obtained form

industrially.

  b) The autoclave is then deaerated. For this purpose, the invention proposes

  several different possibilities in advantageous embodiments.

  One possible de-aeration mode is to rinse the autoclave with steam. The water vapor then expels the air present between the different particles of the charged material. Steam rinsing

  water can be carried out at atmospheric pressure.

  Another embodiment consists in undertaking the deaeration by means of a device for evacuating, such as for example a vacuum pump. The effect is the same, the air present between the particles of solid matter

is evacuated by aspiration.

  According to a third embodiment, the procedure is such that the autoclave is first supplied with water vapor, establishing a pressure of approximately 2 to 3 bars - but this pressure may possibly be greater. In doing so, the air present between the particles is compressed, and at first it does not escape. The lowering of the pressure which is then achieved makes it possible to pull the air from between the particles, and to evacuate it at the same time. The lower the pressure (which can be reduced to atmospheric pressure), the more energetic the deaeration. Tests have shown that by establishing in the first step a pressure of 2.5 bars, a subsequent expansion to about 1.5 bars is sufficient to obtain almost total deaeration. Tests have also shown that it is generally sufficient to evacuate at most about 70%, in order to extract enough air to then have a sufficient porous space to be able to guide completely and uniformly in the cavities the pressurized water vapor which is then fed. c) As indicated above, the autoclave is then fed with steam under pressure by establishing in the autoclave a temperature of between 105 and 180 ° C., in a saturated steam atmosphere. The temperature is preferably between 105 and 1500G C, and even better

between 115 and 130 C.

  In doing so, the saturated vapor pressure is, at a temperature of

 C for example, 2.8 bar.

  d) The humid atmosphere under pressure at the indicated temperatures is maintained until the conversion of the dihydrate to the alpha hemihydrate is achieved if possible totally. As a general rule, we can not obtain a transformation at 10096. The term "total transformation" must be understood as designating the most complete transformation possible by

example at 98.6 and above.

  For an autoclave in which are for example loaded three baskets each containing 1m3 of residual material, the holding time of

  Hydrothermal conditions is approximately 1.5 hours.

  e) The pressure in the autoclave is then lowered to the pressure

normal.

  f) The alpha hemihydrate can then be removed, and finally, according to step g), it can be released by drying the residual water adhering to it (about 20 to 25% by weight from the dehydration , water from

  condensation and introduced moisture).

  Unlike the processes according to the state of the art, it is therefore impossible to perform separate dehydration processes before drying, because the material has a much lower humidity. According to a particularly advantageous embodiment during the manufacture of alpha hemihydrate in large autoclaves and when the capacity of the dryer is possibly limited, the invention proposes to leave at least partially the alpha hemihydrate first. formed in the autoclave as a result of the lowering of the pressure. So do not remove

  the amount that can be directly dried in the dryer.

  The material remaining in the autoclave is then, in order to prevent any rehydration during its residence time in the autoclave, maintained under a humid vapor atmosphere at a temperature of 60 to C, and preferably about 900C. The autoclave thus serves in this phase of the storage container method, whereas according to the state of the art, the storage must always take place in heated silos. it

  constitutes a new advantage of the process according to the invention.

  In order to prevent rehydration, the subsequent drying, which may for example be carried out in tubular dryers, drying drums, heated coils, cyclones or other similar apparatus, is undertaken at a temperature above 80 C and preferably between 80 and 150 C. The drying temperature is at best between

and 130 C.

  In addition, with a view to a further saving of energy, the invention proposes to reduce the heat lost from the dryer in the autoclave following the lowering of pressure according to step e). Of course, fresh steam or residual steam from any

  another process or hot air.

  The invention will now be explained in more detail with the aid of

  different examples of realization.

  In view of the practical use of a plaster, it is on the one hand the properties of the hardened plaster body such as strength, adhesion and specific weight which are decisive, but on the other hand also the behavior of the plaster paste during solidification, and in this context the amount to be spread, the resistance to bending tensile and the

compressive strength.

  In the tests, flue gas desulphurisation plasters from different sources were used, with a moisture content of between 6 and 2390 by weight and one part of calcium sulphate dihydrate.

  greater than 90% by weight, and a fineness of less than 100 μm.

  In the series of tests A, the process was carried out without deaeration of the autoclave, and in the series B with deaeration of the autoclave by rinsing with water vapor. In both cases, the autoclave treatment lasted 90 minutes at 130 ° C. (with a corresponding pressure of about 2.8 bar). The following results were obtained: Test Series A Sample Moisture Weight Quantity Resistance Resistance superfic. volumic to the tract. to the material of the charge. spread by bending bulk starting compression (3) (4) (4)

(1) (2)

A 8 0.870 196 5.2 14.3

B 6 0.990 211 6.3 21.0

C 23 0.880 184 7.8 22.0

  D il 0.815 175 8.8 23.0 Test Series B Sample Humidity Weight Quantity Resistance Superfic. volumic to the tract. to the material of the charge. spread by bending bulk starting compression (3) (4) (4)

(1) (2)

A 8 1.090 254 8.0 27.2

B 6 1,065 265 11.6 41.2

C 23 1.10 280 14.1 45.6

D 11 1,250 310 15.2 48.3

  (1) in weight percent (2) in g / cm3 (3) in g / 1005g H20 (4) in N / mm2 The comparison of the results of the series of tests A with those of the series of tests B shows that the alpha hemihydrates manufactured with the process according to the invention provide about a doubling of flexural and compressive tensile strengths. This is due among other things to obtaining a compact and particularly homogeneous crystalline form

  (almost exclusively in the form of bars) of the semi-

  Alpha hydrate. The samples treated according to the series of tests A have on the other hand a characteristic form of fibrous and acicular crystal, with

important differences.

  As described, the process according to the invention can be carried out in conventional autoclaves. It should also be noted that the process according to the invention can of course also be used for the dehydration of natural plasters in alpha hemihydrate, even if in this case the problems described in introduction encountered with the state of the art are not present to the same extent, since in the case of natural plaster, in the form of "plaster cartilage"; the product is discharged into the autoclave in the form of large, compact pieces which form pores of large volume between them, by means of which a uniform temperature transmission can be obtained more easily during

  heating the autoclave by introducing steam.

  In addition, the subject of the invention is also a mixture of mortar based on an alpha configuration calcium sulphate hemihydrate manufactured according to the method described above and of one or more residual substances of a process of spray adsorption desulfurization, as well as

  than possibly other additives and aggregates.

  It is therefore a mixture of mortar based on two residual materials of the desulfurization process which, advantageously combined with one another, make it possible to produce an advantageous mortar which can be

  especially used in underground applications, such as "mine" mortar.

  It is mainly in the mining industry that there is an urgent need for economic mortars, which are needed in large quantities. The qualities of known cements are not suitable because of their relatively high price due to the large proportion of Portland cement clinker. For mining use, for example as a filling material behind structures, mortars and concretes do not need to present the resistances that cements, for example, reach. In addition, because of the sometimes long transport distances, it is necessary that the corresponding mortars or concretes exhibit an adjustable hardening and solidification behavior, in order to ensure that the

easy and reliable.

  The aforementioned mortar mixture can meet these requirements, and it also contributes to the recovery of the residual materials that are produced

  in considerable quantities during the desulfurization of the flue gases.

  Residual substances from the spray adsorption desulfurization process should be present in this mixture in a content of between 10 and 60% by weight and preferably between 3% and 50% by weight.

  by weight, relative to the totality of the mixture.

  Residual substances, which may also contain some of the fly ash produced during combustion, consist mainly of calcium sulfate dihydrate and / or calcium sulphate hemihydrate and / or calcium sulphate and / or calcium sulphite hemihydrate. We prefer a mixture of mortar

  mainly calcium sulphide hemihydrate.

  The share of fly ash possibly combined in the residual substances can be supplemented by the separate addition of fly ash, ie a new residual product. The fly ash also serves as a hydraulic activator and further improves the properties

resistance of the mortar.

  It is particularly desirable to also add coarse shot, called "sweep shot", such as limestone chips or gravel, preferably in a grain size of between 1 and 3 mm, in order to prevent deposits in the transport routes during pneumatic transport and accumulations in the outlet region during impregnation with water. Finally, it is also possible to add to the mortar mixture other additives, such as fluidifiers or retarders, as for example

  for example surfactant derivatives, lignin sulphonates or the like.

  Advantageously, the calcium sulphate half-hydrate content alpha configuration is equal to 40 to 90% by weight, and preferably from 50 to 96 relative to the totality of the mixture, and the fluidizing additive and / or regulating the solidification of the mortar mixture after addition of water is

  between 0.1 and 1% by weight relative to the totality of the mixture.

Claims (21)

  1. Process for the manufacture of calcium sulphate hemihydrate in alpha configuration from an industrial residue based on calcium sulphate dihydrate, characterized in that it comprises the following steps: a) loading of the residual material into an autoclave b) deaeration of the autoclave c) supplying the autoclave with pressurized water vapor, setting in the autoclave a temperature of between 105 and 180 ° C. d) maintaining the hydrothermal conditions according to c) up to the total conversion of calcium sulphate dihydrate to alpha-configuration calcium sulphate hemihydrate (so-called "alpha hemihydrate") e) lowering of pressure in the autoclave to normal pressure f) removal of semi alpha -hydrate obtained g) elimination, by drying, of the residual water adhering to the alpha hemihydrate.   2. Method according to claim 1, characterized in that   deaerating the autoclave according to step b), the latter is rinsed with water vapor.   3. Method according to claim 2, characterized in that the rinsing with   the water vapor is carried out at atmospheric pressure.   4. Process according to claim 1, characterized in that   deaerating the autoclave according to step b), the latter is evacuated.   5. Method according to claim 4, characterized in that the vacuum is done by means of a vacuum pump.   6. Method according to claim 1, characterized in that the deaeration of the autoclave according to step b) takes place by means of the following partial steps, which follow one another for a short time: bl) supply of the steam autoclave of water, establishing a pressure of approximately 2 to 3 bars,   b2) lowering the pressure by at least 0.9 bar to about 1.1 to 1.8 bar.   7. Method according to any one of claims 1 to 6, characterized   in that the temperature according to step c) is set between 105 and 150 ° C, and preferably between 115 and 130 C.   8. Process according to any one of claims I to 7, characterized   in that the residual material used is a calcium sulphate dihydrate support from flue gas desulfurization (called   "flue gas desulphurization plaster"). l1   9. Process according to any one of claims I to 8, characterized   in that the residual material is added to the autoclave before it is charged with mineralizers, in particular polycarboxylic acids, in particular succinic acid, malic acid, maleic acid, citric acid, methylsuccinic acid or their salts and / or sulphates   inorganic, potassium sulphate and aluminum sulphate in particular.   10. Method according to any one of claims 1 to 9, characterized   in that the alpha hemihydrate obtained, before being removed from the autoclave according to step f), remains for a given time in the autoclave maintaining a humid vapor atmosphere at a temperature of 60 to  C, and preferably 90 C.   11. The method of claim. 10, characterized in that the wet vapor atmosphere is established by supplying fresh steam   or residual and / or fresh or residual hot air drying.   12. Method according to any one of claims 1 to 1i1,   characterized in that the drying according to step g) is carried out at a temperature between 80 and 150 C, and preferably between 100 and oc. -   13. Calcium sulphate half-hydrate of alpha configuration and of compact crystalline form of the bar kind, characterized in that it is   manufactured by the process of any one of claims I to 12.   14. Mortar mixture, characterized in that it comprises alpha-configuration calcium sulphate hemihydrate according to claim 13, one or more residual substances of a spray adsorption desulfurization process, and possibly 3. other additives and aggregates: 15. mortar mixture according to claim 14, characterized in that the residual substances from the spray adsorption desulfurization process are present. in a content of between 10 and 60% by weight and preferably between 30 and 50% by weight, relative to the totality of the mixture. 16. Mortar mixture according to claim 14 or 15, characterized in that the residual substances consist for the most part of calcium sulphide hemihydrate.   17. Mortar mixture according to any one of claims 14 to   16, characterized in that the calcium sulphate half-hydrate content of alpha configuration is 40 to 90% by weight, and preferably 50 to 70% by weight   by weight, relative to the totality of the mixture.   Mortar mixture according to one of Claims 14 to   17, characterized in that there is added sweep shot, preferably splinters of limestone rock. 19. Mortar mixture according to claim 18, characterized in that the sweep shot is present in a grain size of between 1 and 3 mm.   20. Mortar mixture according to any one of claims 14 to   19, characterized in that fly ash is added thereto.   21. Mortar mixture according to any one of claims 14 to   , characterized in that there is added, preferably in an amount of between 0.1 and 1% by weight relative to the totality of the mixture, a fluidifying additive and / or regulating the solidification of the mortar mixture after addition of water.