LU83742A1 - IMPROVEMENTS TO CALCINATION OF CALCIUM SULFATE DIHYDRATE - Google Patents

Description

Improvements in the calcination of calcium sulphate dihydrate.

The present invention relates to an improvement to the method and to the apparatus for calcining calcium sulfate dihydrate or gypsum.

When calcining the dihydrate to hemihydrate, any tendency to produce insoluble anhydrite at the expense of the hemihydrate is a disadvantage since in some applications this leads to a reduction in the effective use of mineral gypsum subjected to the treatment. Under certain conditions, this tendency can be observed in continuous calcination processes based on direct heating of the gypsum by hot gases, where the mineral supplied is calcined and very quickly transformed into hemihydrate.

The present invention provides a means for reducing the production of insoluble anhydrite, in a continuous calcination process, by direct gas heating of a gypsum bed, to levels which are, for example, typical of the production of a boiler operating without direct action of hot gases on the mineral used.

According to the present invention, it is intended to spray moisture in the gas phase above the bed, for example in the exhaust gas space in or near the top of the calcination container or in the tube through which the hot gas is introduced into the bed, or in these two places.

The invention will be better understood on reading the description below which is made with particular reference to the production of calcined gypsum according to patent GB 1,488,665 and published patent application GB 2,043,219A. Although the detailed description of the invention relates to these two methods and apparatuses, it will be understood that the invention can be applied to other methods and apparatuses for direct calcination of calcium sulphate, and more particularly to equipment calcining dihydrate. calcium sulfate quickly and by direct heating.

GB Patent 1,488,665 describes a calcination process 2 for calcium sulphate dihydrate in which the dihydrate is heated in a calcination container by heat applied indirectly through the external walls of the container and further heated by the direct introduction of non-reactive hot gases, and preferably hot gaseous combustion products, into the interior of the mass of dihydrate contained in the container via a tube generally oriented towards the bottom from the top of the container and provided with at least one opening 10 in its bottom region which is immersed in the mass of the material. For maximum production and to obtain all the advantages of the process, it is preferred to operate the calcination vessel continuously, the calcined material being discharged via a weir or any other level control device. .

The adoption of this development makes it possible to increase both the thermal efficiency and the production rate of a conventional boiler. However, when the rate of heat supplied by direct introduction of hot gases is high, the above-mentioned tendency has been observed to produce insoluble anhydrite at the expense of the hemihydrate.

Although a variety of hot gas sources or hot gaseous combustion products can be used, it is currently preferred to use a fuel burner located at the top of the calcination vessel or tube. Advantageously, the burner can be located at the upper part of the gypsum calcination boiler, the tube reaching into the bed. The combustion gases are preferably distributed through holes made in the side walls of the lower region of the tube inside the bed, while the lower region of the tube is preferably open. In the preferred embodiment of the invention, water or any other aqueous liquid (or steam) is introduced into the burner tube at a controlled rate.

The flow of water or aqueous liquid should be adjusted by means of a trial and error technique to 3 "obtain the best results and avoid destabilization of the burner flame in the tube. In a typical installation, the most suitable flow rate is 4.5 liters per minute.

The published GB patent application No. 2,043,219A describes a method and an apparatus for heat treatment of a particulate material, that is to say of a powdery or granular material, and especially suitable for calcination, in which all the necessary heat can be sent directly to the material to be calcined, which makes it possible to eliminate a large part of the refractory brick installation of conventional boilers and to thermally insulate the installation, so as to increase thermal efficiency and reduce investment and maintenance costs.

According to this request, the method comprises the steps consisting in introducing the material to be calcined in the container, in limiting the material to the lower region of the container in the vicinity of the outlet of the hot gases, in particular by using a container of reduced cross section in its 20 lower region, introduce a hot gas down through the container until it establishes direct contact with the material in the bottom region, the material located at the bottom being simultaneously heated and circulated and l Agitation and the resulting heating will propagate from the lower region into substantially all of the material in the container, and to remove the heat treated material from the container. It is considered that in most cases the hot gases provide all the heat necessary for treatment in the container, although the wall can be heated to a certain degree without departing from the principles of demand.

According to the present invention, water or any other aqueous liquid or vapor is also introduced in this case, either at the level situated above the material contained in the container, or, preferably, in the heating tube: , hot gases.

In the accompanying drawings: / Figure 1 is a schematic sectional view of a

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4 calcination boiler according to GB patent 4 188 665 modified according to the present invention, FIG. 2 is a longitudinal section of a burner tube suitable for the boiler of FIG. 1, and FIG. 3 is a schematic section of a calcination container according to published GB patent application No. 2 043219A, modified according to the invention.

As shown in FIG. 1, a calcination boiler 10 comprises a container 11 surrounded by a heating chamber 12 provided with a burner 14 or any other supply of hot gas for heating the material contained in the container indirectly by conduction by through the walls, and especially through the bottom of the container. Conventional transverse carnals 15 facilitate the transfer of heat to the material contained in the container. The cover 16 of the boiler is provided with openings 17 intended for the introduction, preferably continuous, of calcium sulphate dihydrate ground or in any way transformed into powder, and 18 for the extraction of the exhaust gases in management of a dust collector. A stirring device or mechanical agitator 19 is driven by a shaft 20 passing through the cover. A weir or discharge tube 21, protected by a baffle 22, is provided for discharging the calcined material according to patent 25 GB No. 1,018,464 and there is also provided a conventional lower outlet 23 with valve.

In order to apply the principles of patent GB 1 488 665, a vertical tube 24 is placed inside the container and it is provided with a burner to which fuel and air are sent via respective pipes 25 and 26. The hot combustion gases which are produced by the burner are introduced directly into the mass to be calcined by means of openings made in the lower end 27 of the tube.

According to the present invention, water, steam or an aqueous solution is sprayed into the calcination vessel over the mass of material being calcined. Examples of the positions where the water nozzles / U ~ \ »5 can be located in the boiler are indicated at 30 at the top of the hot gas tube 24, at 31 towards the lower region and above the openings of the tube, and at 32 in the free space situated above the level of the mass in the container 11 proper.

FIG. 2 shows a preferred construction of the hot gas tube 24 and the equipment associated therewith. At the head of the tube, above the level 34 of the boiler cover, there are provided the gas and air inlets 25 and 26 10 respectively, as well as water supply pipes 35.

A radial type burner 36 surrounded by a conical skirt 37 is fixed inside the tube 24 and connected to the gas inlet. In the embodiment shown in the drawing, the burner is arranged just below the level 38 of the bed 15 of material contained in the container 11, but in other embodiments, the burner can be positioned in a more high, and for example immediately below the level of the cover 34 of the boiler. The bottom of the tube 24 is provided with lateral openings 39 and the lower end of the tube is completely open.

Two spray nozzles 40 with hollow cone are mounted inside the tube 24 approximately at the level of the burner 36 with a view to the introduction of moisture according to the present invention. The nozzles are connected by pipes, supplying water 35 to 25 with a suitable supply of water under pressure.

In operation, the gypsum rendered pulverulent is introduced continuously through the opening 17 into the container 11 where it is partially heated indirectly by the heating chamber 12. At the same time, fuel and air introduced into the tube 24 provides additional heat by direct introduction of hot combustion gases, which also agitate the material in the container. The contents of the container are also agitated and fluidized by the water vapor formed during the calcination, while the material contained in the container is further agitated by the stirring device 19. The calcined product is poured continuously by the discharge tube 21, the baffle 22 avoiding direct access of material possibly ts \ 6 not calcined between the inlet 17 at the outlet 21. Water or an aqueous solution is sprayed in the gases contained in the tube 24 by means of the spray nozzles 40, which has the effect of reducing the formation of insoluble anhydrite in the boiler to a negligible level.

With the help of another example which is illustrated in FIG. 3, the invention will be described with particular reference to the production of calcined gypsum according to published patent application GB No. 2,043,219A.

As shown in FIG. 3, a container 45 of inverted conical shape comprises a rounded bottom 46 of limited surface area and it comprises an upper part 47 which is in turn closed by a cover 48 provided with a supply pipe 49 intended to the material to be calcined, such as powdered gypsum 15, and an exhaust gas outlet pipe 50 connected to a dust collector (not shown). A heating tube 51 which is described in more detail below also passes through the cover 48 to reach the interior of the container. The gypsum supply pipe 49 is provided with a metering valve in the form of a rotary supply device 52 which is connected to a gypsum compartment 53. The container 45 and the upper part 47 are suitably insulated, as indicated in 54.

The normal level of the pulverulent material contained in the container when it is operating is indicated at 10. An outlet for the calcined material is provided in the form of an external weir 55 connected to an outlet pipe 56. A lower discharge door 57 to valve is also provided.

The heating tube 51 extends substantially downwards along the central vertical axis of the conical container 1. It is open at its lower end 58 and it ends in the vicinity of the bottom 46 of the container. The tube is also provided in the lower part of its wall with holes 61 arranged symmetrically and helping to facilitate the distribution of the hot gases in the material being calcined. The distribution of the gases emerging from the tube 51 is further improved by an upright cone or any other projection 62 / made of refractory material which is disposed on the bottom 46 of the h.

7 container directly below the opening 58 of the tube.

Gaseous fuel such as natural gas is sent through the pipe 63 to a gas burner 64 of the mixing nozzle type located inside the tube 51, approximately at the level of the material contained in the container. Air is sent separately to the burner by an air hose 65, coming from a fan 66. The hot gaseous products of combustion descend into the tube 51 and are evacuated by its open end 58 and the holes 61. Generally , an auxiliary air supply to the heating tube is not used in this device.

According to the present invention, water or an aqueous solution, steam or any other form of humidity is vaporized in the tube 51, either in the upper region of the container as indicated in 30, or lower in the tube. , for example near the level of the material 10 or near the burner 64, as indicated in 31. As a variant and additionally, it is possible to spray moisture in the free space of the container in the vicinity of the outlet of the -20 gas exhaust 50 or at least partially in the outlet pipe itself, as indicated in 32.

Λ 25 30 35

Claims (15)

1. A method of calcining gypsum in which a bed of powdered gypsum is heated directly by introducing hot gas into the bed so as to maintain it at a temperature suitable for the production of calcium sulphate hemihydrate, characterized in that 'Moisture is sprayed into the gas phase, thereby reducing the formation of insoluble anhydrite. 2. Method according to claim 1, characterized in that the moisture is sprayed into the hot gas before the latter contacts the gypsum. 3. Method according to claim 1, characterized in that the moisture is sprayed in a region located above the gypsum bed occupied or crossed by the exhaust gases from the bed. 4. Method according to claim 2, characterized in that the hot gas is introduced into the gypsum contained in a container via a tube extending generally downward from the top of the container and opening into from the bottom of the container, the humidity 20 being sprayed inside this tube. 5. Method according to claim 4, characterized in that the hot gas is supplied by a burner mounted inside the tube, and in that the moisture is sprayed at substantially the same level as the burner. 6. Method according to any one of claims 2 to 5, characterized in that the gypsum is contained in a container which is additionally indirectly heated by thermal conduction through the walls of the container. 7. Method according to any one of claims 4 or 5, characterized in that the gypsum is contained in a container whose lower region is shaped so as to direct the gypsum towards the vicinity of the lower end of the tube, the gas hot from the stirring tube and heating all the material in the container. 8. Method according to any one of claims 6 // - Â * 9 or 7, characterized in that the gypsum is introduced continuously into the container and overflows or is continuously removed in the calcined state. 8 " 9. Apparatus for calcining gypsum, comprising a calcination container (11) for containing a bed of gypsum to be calcined and means for directly introducing a hot gas into the gypsum bed, characterized by means (30, 31, 32) to spray moisture into the gas phase in or near this container. 10. Apparatus according to claim 9, characterized in that the container (11) is provided with a tube (24, 51) for the introduction of hot gas, generally extending towards the Das inside the container and opening in its lower region, and in that at least one humidity spray nozzle (30, 31) is arranged inside the tube. 11. Apparatus according to claim 10, characterized in that a burner (64) is mounted inside the tuoe (24, 51. to supply a source of hot gas, and in that at least one nozzle of the water (31) is fixed inside the tube approximately at the level of the burner. 12. Apparatus according to any one of claims 9 to 11, characterized in that the container (11) comprises thermally conductive walls and is surrounded by a heating chamber (12) for the indirect heating of the contents of the container. 13. Apparatus according to any one of claims 9 to 11, characterized in that the container (11) has a lower part (45) shaped to direct the contents of the container 3U pient in operation towards the lower end of the tube (51). 14. Apparatus according to any one of claims 9 to 13, characterized in that it comprises means (17, 53, 52, 49) for continuously introducing powdered gypsum and a discharge pipe or weir (21, 55, 56) by which the calcined material is discharged continuously. 15. Apparatus according to claim 9, characterized in that at least one moisture spray nozzle (32) is mounted in or near the container at a level which is located above the gypsum bed for the operation, in a region occupied or crossed by exhaust gases? ment from the bed. b