Treating process for the damp ( moist ) gypsum

The invention relates to a process for the thermal treatment of wet gypsum, by which the raw material is treated with hot gas, in a fluidized layer, located in a first fluidized bed, with heat exchange surfaces, in continuous flow and in another hot gas fluidized layer, located in a second fluidized bed, with continuous flow heat exchange surfaces.

Methods of heat treatment of moist hydrates are known. Thus, the calcination of wet plaster is known. Wet gypsum (calcium sulphate dehydrated) can be dried superficially, at temperatures up to 9 ° C, without removing the crystallization water. At temperatures up to 145 ° C, the crystallization water is extracted, respectively, dehydration, to obtain a semi-hydrate.

In the heat treatment processes, wet plaster can also be used, resulting in sulfur removal plants from the flue gases.

Depending on the partial pressure of the water vapor, at the same time a change of or β of the semihydrate takes place. Above this temperature, depending on the temperature level, by a total elimination of the crystallization water, the modification in anhydrite, starting with the so-called gypsum A III, a very reactive component, with a very short outlet time, followed by gypsum A II. S with slow, hard soluble reaction, as well as gypsum A II u, which is inert. These behavioral differences, in reaction, are caused by the superficial structure, modified as a result of the heat treatment.

Therefore, in order to obtain a calcined gypsum, with a constant and / or pure composition of the respective phases, it is important that the temperature variation can be precisely controlled in the manufacturing process. Overheating and inhomogeneities should not occur during heat treatment, phenomena that cause undesirable phase changes in the case of material particles.

This is true for all thermal phases, so also for greases.

There is no process for heat treatment of moist hydrates, in which the principles stated above can really be respected. Known processes have separate drying and calcining steps. For the drying step, gas-fired tubes or combinations of heated mixers and gas-fired tubes are used, which are operated with combustion gases, or indirectly heated gases, above 2OO ° C. For contact surface heating, conventional heat carriers, for example oil or steam, with a temperature of 15 ° C are also used. It is not possible to eliminate the overheating of the material by too long contact with the hot gas streams, respectively with the heating surfaces, forming here, on drying, unwanted phases, for example gypsum forms A III, A II.

For calcination, the so-called boilers, drums and rotary drums, with heating surfaces (steam pipes), as well as various types of ovens are used. Drums and ovens with direct heating are part of the conventional technique of burning gypsum and are suitable, only for unpolished material, in pieces.

Boilers and rotary drums, with heating surfaces, are supplied with granulated, fluidizable gypsum powder, gypsum which can also be obtained from sulfur removal plants from flue gases.

For the mentioned devices, it is characteristic, either the direct heating with hot gases (combustion gases, respectively air) having, usually, over 400 ° C and / or the incomplete fluidization in the area of ​​the heating surfaces. In this way, as in the case of dryers, it is possible to reach the local overheating and an uncontrolled formation of the plaster with several phases. In addition, the flue gases are not inert and a controlled atmosphere cannot be achieved (for example, a partial pressure of H₂0). These disadvantages of calcinators are sufficiently well known and are therefore the subject of proposals for improvement. In practice, these disadvantages of the processes are compensated, by subsequently influencing, the qualities of the product, by different additions, by the so-called adjustment materials, respectively, by additional phases of the processes (milling, cooling).

The known proposals have the purpose of eliminating from the flue gases the high temperature, harmful, by carrying out in advance, an indirect heat exchange, the use of fluidized bed appliances, to intensify and homogenize the heat transmission, as well as to prevent the contact between the flue gases. combustion and the product concerned.

Thus, for example, a patent for FR 1338126 describes a process for heat treatment of moist hydrates in a fluidized bed apparatus, which operates with hot air, which can only solve the fluidization problems in the feeding area, by means of a mixer so that the danger of local overheating is maintained, in particular along the flue gases passing through the layer. These disadvantages are avoided by a process described in patent DE 3721421, a process carried out in a fluidization bed with inert granulate filling, (sand) in a closed circuit gas stream. In a patent DE 2622994, a fluidizing bed apparatus is described, in which an attempt is made to improve the inhomogeneous quality of the product, through a uniformization of the residence time, with the help of chambers. However, these devices have the disadvantage that in order to obtain sufficient drying efficiency and corresponding efficiency, high hot gas temperatures must be worked, with the known dangers of overheating.

In a patent GB 2027859 A kettle-like apparatus is presented, in which the combustion gases are cooled by heat exchanging surfaces in the product layer, before being used for (partially) fluidization. In DE 3738301, this principle of the indirect heat exchanger is repeated. On the other hand, for fluidization, hot gas is used, indirectly heated, not combustion gases, trying to homogenize the dwell time through differentially fluidized chambers. And in the case of these two processes, the danger of overheating persists, on the walls that transmit the heat, especially in the case of partial fluidization, respectively differentiated. In addition, all the processes described above, except for the fluidized bed with granulated material (DE 3721421) require a powder of dihydrate, dried by conventional procedures.

The problem, which is solved by the invention, is to improve the process of heat treatment of moist hydrates, in order to achieve the desired result in a controllable way, regarding the elimination of crystallization water. In particular, calcined gypsum, with a constant and / or pure phase composition, in which the non-homogeneous treatment, to a large extent, which results in obtaining a homogeneous product, is to be controlled in a controlled manner. qualitative view.

The process according to the invention consists in the evaporation of surface water from the material subjected to treatment, at a usually low temperature level, in a first fluidized bed, which is in a first fluidizing device. Then, at low temperature, the crystallization water from the treated material, which is in another fluidized bed, is located in a separate fluidizing device. The heat, to be used, is introduced alternatively, through the fluidization gas and in multiple ways, through heat exchanger surfaces.

The material subjected to the heat treatment is dehydrated before the first fluidizing device and the resulting dehydrated product is mixed with some of the material already treated in the first fluidizing device.

By the process according to the invention, the disadvantages mentioned above are avoided, in the case of wet gypsum calcination. Both the drying and the removal of the crystallization water are performed separately in a fluidized bed apparatus, without the danger of overheating. This is achieved by a completely homogenous fluidization, by means of a hot gas, indirectly heated, at low temperature. A significant amount of energy is transferred, through the heat exchanger surfaces, into the fluidized layer, which is heated by different agents, chosen from saturated steam, oily thermal agent, at a low temperature level, preferably at 2DO ° C. A very advantageous constructive composition of the rectangular-based appliances, ensures a reduced spectrum of the time of stationing of the material subjected to the heat treatment and a pronounced profile of the temperature and humidity, without the use of rooms with different fluidization. Due to this, a relatively cold input area and the appropriate heating are achieved, the surface properties of the crystals are advantageously influenced.

The fluidized bed consists of the material to be treated, namely the plaster granulate. No auxiliary material or other mechanical auxiliary means (eg mixer) are required. The safe operation mode is obtained by a large depth of the fluidized bed, of 0,5 ... 1,5 m. In order to be able to dry and hydrate with high humidity, it is necessary to mix such materials before entering the drying area with recycled material, superficially dry to a non-critical humidity, distributing the food in such a way that the surface loading specifies not to disrupt fluidization. This can be advantageously achieved by several supply points and / or by the provision in the layer of embedded impulse braking or distribution elements.

Adjusting the temperature of the fluidizing bed can be done either through the feed flow of the product or through the temperature of the thermal agent. In the case of wet gypsum, a superficial dry dihydrate is obtained, with total crystallization water content at 9D ° C (product temperature). With this initial material, any desired gypsum quality can be obtained, only by the variation of the temperature at the output, which can be kept constant, within limited limits. Thus a pure semi-hydrate B, a two-phase gypsum, a semi-hydrated β and a different content of gypsum A III, depending on the temperature, can be obtained, as well as a multi-phase gypsum, with semi-hydrated β parts of gypsum A. III and A II up to a pure A II product. Integrated into the burner, at its output end, a cooling zone can be found, which will cool the product to moderate temperatures, offering, in addition, the possibility, by a controlled addition of humidity, to realize a blocking of product.

In the following, an embodiment of the process according to the invention is described, in relation to the figure, which schematically represents an installation, for carrying out this process, representing the calcination of wet plaster.

A mechanically dehydrated gypsum plaster from a flue gas discharge plant is mixed, after discharge from a wet material silo Ί, into a mixer 2, with a previously dry dry dehydrate, up to upon reaching a non-critical moisture, so that the granulated product can be well fluidized. This material is introduced, via a transport device 3, at two feed points, into a first fluidized bed apparatus (dryer A). The fluidizing air is heated in a heating register 5 to 15 ° C. □ a significant amount of energy is introduced into the fluidized layer of dryer A via the heat exchanger system 4. The air that is discharged from dryer A is separated from the dust in a filter or cyclone 6.

The dust is transported along with the surface dry hydrate pneumatically from dryer A via a pipe 7 into a dry material silo 8. Part of this quantity is diverted, passing through a transport screw 9, to reach as recyclable material, in the mixer 2. The rest of the material in the dry material silo reaches, through a metering device 10, a second fluidized bed device, in a burner B. The fluidizing air, for the hot part of the fluidized bed device B, is heated by means of a register 11, depending on the desired degree of calcination. The fluidizing air, for the cooling zone integrated in the fluidized bed device B, is not heated. A significant amount of energy is introduced into the hot part of the burner B by means of a heat exchanger system 12, according to the degree of calcination sought. From the cooling zone, a significant amount of energy is discharged through a heat exchanger system 13. The air discharged from burner B is blown into a cyclone 14, and the powder is re-introduced, depending on the degree of calcination of the material, in a suitable place, in the material layer. The chilled gypsum, whose calcination is completed, reaches, through pneumatic pipes 15, in a silo 16, for the finished product and can be extracted from it for further processing.

The fact that the embodiment was designed for the dihydrate obtained from a sulfur removal plant from the flue gases, for which the process is particularly indicated, does not exclude the application of the process for calcining other forms of gypsum dihydrate. The process can be applied with the advantages described. And other moist hydrates can be processed according to the invention, in the manner indicated, with the advantages mentioned above.