Method of thermally treating wet hydrates

The subject of the invention is a method of thermal treatment of moist hydrates, in which the surface water from the fluid gas fluidized layer of this material in the first fluidized bed with embedded heat exchange surfaces is evaporated at a low temperature level in a continuous process and then the water of crystallization from the fluidized fluid gas of this material in the second fluidized bed with embedded heat exchange surfaces is stripped at a low temperature in a continuous process.

Such a method is e.g. the so-called calcining of wet plaster. Moist gypsum, i.e. calcium sulphate dihydrate, which also includes wet gypsum from REA (from the flue gas desulphurization unit = RauchengasEntschwefelungsAnlagen), can be surface dried at 90 ° C without removing water of crystallization. At a temperature of up to about 145 ° C, the water of crystallization is stripped off, dewatering to a hemihydrate. Depending on the partial pressure of the water vapor, an alpha- or beta-variety of this hemihydrate is produced. Above this temperature, depending on the temperature level, they arise as a result of the total donation of water anhydrate varieties, starting with the so-called A III gypsum, a very reactive component with a short setting time, and continuing through passive, sparingly soluble AII s gypsum and neutral AII in gypsum. These differences in reaction properties boil down to the surface properties changed by heat treatment.

To obtain calcined gypsum with a solid and / or pure phase composition, it is therefore important to be able to accurately control the temperature profile during manufacture. There must be no overheating and heterogeneity during heat treatment, leading to

168 241 case of individual particles of material for undesirable phase transitions. This applies to all thermal stages, and therefore also to drying.

To date, there is no way in which the maximum conditions discussed above can actually be met. Known methods show separate drying and calcining steps. In the drying stage, flow pipes or combinations of heated mixers and flow pipes are used, which are operated by means of flue gas or by means of indirectly heated gases of the order above 200 ° C. Known heat transfer agents are also used for the heating of the contact surfaces, e.g. oil or steam, from about 150 ° C. It cannot be avoided that the material, at least partly, does not overheat due to too long clogging with hot gas streams or heating surfaces and that no undesirable phases such as A III, AII form here during drying.

The calcining process uses so-called boilers, drums, rotating pipes with heating surfaces (steam pipes) and various types of furnaces. Directly fired drums and ovens come from conventional gypsum burning techniques and are only suitable for lump material.

The boilers and steam pipes are fed with fluidized gypsum powder (granulate), such as obtained from REA (from flue gas desulphurization equipment).

Characteristic for these devices are either direct firing with hot gases (flue gas or air) with a temperature usually above 400 ° C and / or incomplete fluidization within the heating surfaces. As a result, as with a dryer, local overheating and uncontrolled formation of multi-phase gypsum occur. In addition, the flue gases are not inert and a controlled atmosphere (e.g. H2O partial pressure) cannot be adjusted. These disadvantages of calcining are sufficiently known and are the subject of improvement proposals. In practice, these inconveniences are compensated for in such a way that due to the different addition of admixtures of so-called setting substances, or as a result of additional operations (milling, cooling), the product properties are adversely affected.

Known proposals aim to remove harmful harmful excessive temperatures by means of a previously attached indirect heat exchange flue gas, use fluidization apparatus to intensify and homogeneize heat transfer and prevent flue gas from coming into contact with the product.

For example, FR-PS No. 1 338 126 describes a fluidized bed apparatus operated with hot air, which, however, fluidization problems in the feed zone can only be solved by means of a mixer, which means that there is again a risk of local overheating, especially in the draft exhaust gas leading through the layer. These disadvantages are prevented by the method according to DE-PS No. 37 21 421, i.e. a fluidized bed filled with an inert granulate, such as sand, and the addition of gas circulation. Patent DE-PS No. 26 22 994 describes a fluidized bed apparatus which attempts to improve the heterogeneous product quality by uniforming the residence time with the help of chambers. This apparatus, as an inconvenience, is accused of operating with a high temperature hot gas, i.e. in the face of the known danger of overheating, in order to achieve sufficient drying performance and efficiency.

GB 2 027 859 A offers a boiler-like apparatus that cools the exhaust gas through the heat exchange surfaces in the product layer before it is used for (partial) fluidization. DE-OS description No. 37 38 301 follows this principle of indirect heat transfer. In addition, indirectly heated hot gas rather than flue gas is used for fluidization, and attempts to unify residence time are achieved through differently fluidized chambers. Also with both these methods there is a risk of overheating on the heat-transfer walls, especially in the case of partial or various fluidization. In addition, all the processes discussed so far, except for the granular fluidized bed (DE-PS Patent No. 37 21 421), require conventionally dried and thus, as already explained, pre-damaged dihydrate powder.

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From US-A-2 485 317 a method for dehydrating gypsum is known, in which the heating of the powdered gypsum is carried out in an apparatus without reaching for indirect heating. At the same time, very finely divided raw material and hot gas are introduced into the drying zone, this hot gas is blown up through this zone to form a fluidized layer of this material, at least partially dried material is introduced into the hydration zone, in which also due to blowing up a hot gas produces a fluidized layer of this material, and at least part of this hot gas from the top of the dehydration zone is led to the bottom of the drying zone. These two zones with fluidized beds are placed one above the other in one apparatus. These zones can also be equipped with pipes through which a heating medium, such as steam, circulates.

US-A-4,585,645 relates to a process for converting hydrated alumina into an anhydrous product that contains at least 10% by weight of α-alumina. This process is carried out at very high temperatures, with no built-in heat exchange surfaces.

The object of the invention is to develop a method of the type discussed in the introduction, in which the method could precisely control the temperature profile during heat treatment and prevent overheating and heterogeneities that lead to undesirable phase changes of the material being treated.

This goal is achieved by the method of thermal treatment of moist hydrates discussed in the introduction, according to the invention in that the first and second fluidized beds are carried out in separate apparatuses and the heat is mainly introduced by means of membrane heat exchange.

This method avoids the disadvantages mentioned above for the example of calcining wet gypsum dihydrate, preferably gypsum from a gas desulphurization device. In a given fluidized bed apparatus, drying or drawing of the crystallization water takes place gently without any, even partial overheating. This is achieved by complete, homogeneous fluidization due to the indirectly heated hot gas at low temperature. A significant amount of energy is transferred to the fluidized layer through the surface of the heat exchangers, which is heated by factors such as saturated steam, an oil heat carrier, optionally with a low temperature level, preferably at, for example, a temperature of about 200 ° C. The particularly favorable structural design of these apparatuses with a rectangular horizontal projection ensures a narrow residence time and an excellent temperature / humidity profile without using variously fluidized chambers. Due to the resulting relatively cool introduction zone and the resulting gentle heating, it has a beneficial effect on the surface properties of the crystals.

The fluidized bed consists of a material to be treated, e.g. gypsum granulate. No auxiliary materials or mechanical aids (such as agitators) are needed. A reliable function is achieved due to the deep layer of the fluidized bed, e.g. 0.5-1.5 m. To allow the drying of also highly wet hydrates, such materials must preferably be mechanically dehydrated and mixed with recycled, surface dried material up to non-critical moisture content before entering the drying stage, and this feed is dispersed in such a way that proper surface loading does not interfere with fluidization. This can be realized in an advantageous way thanks to several places of power supply and / or inhibiting impulses or distracting elements built into this layer.

Fluidised bed temperature control can be carried out either by product processing capacity or by heat carrier temperature. In the case of moist gypsum, a surface dried dihydrate with full crystallization water content is obtained at a product temperature of about 90 ° C. With this starting material, it is possible in the calciner, in this second fluidization apparatus, only by changing the outlet temperature, which can be kept constant in a narrow range, to produce any desired quality of gypsum: pure β-hemihydrate, two-phase gypsum with beta-hemihydrate content and depending on temperatures with different AII gypsum contents and multi-phase gypsum with beta-hemihydrate, A III- and A Il-gypsum proportions up to pure A and gypsum. In this calciner there may be an integrated cooling zone at the discharge end of the calciner in which this product

168 241 is preferably cooled in a fluidized layer with gas and by membrane heat exchange to a mild temperature, and it is possible to carry out product aridization in this zone by deliberately adding moisture.

The following embodiment further discusses and explains the method according to the invention. This discussion refers to the drawing which schematically shows the device for carrying out the embodiment, namely the calcining of wet plaster.

Mechanically dehydrated wet gypsum from the REA device (from the flue gas desulphurization device) from the silo 1 of the moist material together with already dried surface dihydrate is mixed in the mixer 2 to non-critical moisture content so that the granulate is easily fluidized. This granulate through the transmission device 3 is introduced in two places to the first fluidization apparatus A, i.e. to the dryer. Fluidizing air is heated in a heating stage 5 to 150 ° C. A significant amount of energy is fed into the fluidized layer in the dries A through the heat exchanger system 4. The exhaust air from the dryer A is dedusted in the filter or in the cyclone 5. This dust, together with the surface dried dihydrate from the dryer A, is pneumatically sent through a pipe 7 to silo 8 of the material dry. A part of the quantity through the screw conveyor 9 branches off as an add-back to the mixer 2. The rest of this material from the silo 8 of the dry material goes through the dispenser 10 to the second fluidization apparatus, i.e. to the calciner B. Fluidizing air for the hot part of the fluidization apparatus B is heated by heating stage 11 according to the desired degree of calcination. The air introduced into the cooling zone integrated in the fluidization apparatus B is not heated. A significant amount of energy according to the desired calcination purpose is introduced into the hot part of the calciner B through the heat exchanger system 12, significant amounts of energy are removed from the cooling zone through the heat exchanger system 13. The exhaust air from calciner B is dedusted in cyclone 14, and this dust, according to the degree of calcination of this material, is recycled to a suitable place in the material layer. The finished, calcined, cooled gypsum is fed by pneumatic conduits 15 to the silo 16 of the final product and can be taken from there for further processing.

The fact that the embodiment has been conceived for the REA dihydrate, for which this method is particularly suitable, does not preclude the use of this method for calcining also other varieties of gypsum dihydrate types. Also in these cases the method can be used with the described advantages. Other moist hydrates can also be treated according to the invention according to the advantages mentioned above.

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