GB2086874A - Calcining calcium sulphate dihydrate - Google Patents

Abstract

In a method and apparatus for the continuous calcination of calcium sulphate dihydrate (gypsum) to hemihydrate plaster, a bed of gypsum is heated to the required calcination temperature at least in part by means of hot gas introduced into the bed, moisture (e.g. in the form of water droplets or steam) being sprayed into the gaseous phase, for example into the hot gas before contact with the bed and/or into exhaust gas above the bed to reduce the degree of production of insoluble anhydrite.

Description

SPECIFICATION Improvements in calcining calcium sulphate do hydrate This invention relates to an improvement in the method and apparatus for calcining calcium sulphate dihydrate or gypsum.

In calcining the dihydrate to hemihydrate, any tendency to produce insoluble anhydrite at the expense of hemihydrate is disadvantageous since in certain applications, this will reduce the effective use of the gypsum mineral feed. This tendency can be observed, under some conditions, in continuous methods of calcination based on direct heating of gypsum by hot gases, where the mineral feed is calcined to hemihydrate very quickly.

The present invention now provides a means for reducing the production of insoluble anhydrite in continuous calcination with direct gaseous heating of a gypsum bed, to levels which are typical of the production of, for example, a kettle operating without the direct action of hot gases on the mineral feed.

In accordance with the present invention, provision is made for spraying moisture into the gaseous phase above the bed, for example the exhaust gas space in or adjacent to the upper region of a calcination vessel or into the tube by which the hot gas is introduced into the bed, or in both these locations.

The invention will be described in greater detail with particular reference to the production of calcined gypsum in accordance with our British Patent 1 488 665 and with our published UK Specification 204321 9A. Whilst this detailed description of the invention relates to these two methods and apparatus, it should be understood that the invention can be applied to other methods and apparatus for continuously calcining calcium sulphate, and more especially the equipment which calcines calcium sulphate dihydrate in a rapid manner by direct heating.

In our British Patent 1 488 665 a method for calcining calcium sulphate dihydrate is described in which the dihydrate is heated in a calcining vessel with heat applied indirectly through the external walls of the vessel and heated additionally by the direct introduction of nonreactive hot-gases, preferably hot gaseous combustion products, into the interior of the mass of the dihydrate within the vessel through a tube extending generally downwardly from the top of the vessel and provided with at least one opening in its lower region, which is immersed in the mass of the material. To achieve maximum output and realise the full advantages of this method it is preferred to operate the calcination vessel continuously, calcined material being discharged over a weir or other level-control device.

Adoption of this development increases both the thermal efficiency and throughput of a conventional kettle. However, at higher input rates of heat supplied by the direct introduction of hot gases, the aforementioned tendency to produce insoluble anhydrite at the expense of hemihydrate has been observed.

Although a variety of sources of hot gase or gaseous combustion products can be used, it is at present preferred to use a fuel burner located at the top of the calcination vessel or tube. The burner may be conveniently located on top of the gypsum calcining kettle, with the tube leading into the bed. The combustion gases are preferably distributed through holes in the side walls of the lower region of the tube within the bed, while the lower region of the tube is preferably open-ended.

In the preferred practice of this invention, water or other aqueous liquid (or steam) is introduced into the burner tube at a controlled rate.

The rate of water or aqueous liquid supply should be adjusted by a trial and error technique to achieve the best results and to avoid destabilisation of the flame of the burner in the tube. In a typical installation a suitable rate is 4.5 litres per minute.

Our copending Application 80 00 250 (Published Specification 2 043 21 9A describes a method and apparatus for heat-treatment of particulate, i.e. powdered or granular, material and especially suitable for calcination, in which all the necessary heat can be supplied directly into the material being calcined, thereby making it possible to dispense with much of the refractory brickwork associated with conventional kettles and to thermally lag the system, so increasing the thermal efficiency and reducing capital and maintenance costs.

According to that application the method comprises introducing the material to be calcined into a vessel, restricting the material in the bottom region of the vessel to the vicinity of the hot gas outlet, notably by using a vessel with reduced cross-section in its lowest region, introducing hot gas downwardly through the vessel into direct contact with the material in the region of the bottom, whereby the material at the bottom is simultaneously heated and circulated and the resulting agitation and heat extends from the bottom region through substantially all the material in the vessel, and withdrawing heattreated material from the vessel. It is envisaged that in most cases the hot gases will provide all the heat required for treatment in the vessel, although some degree of wall heating may be provided without departing from the principles of the application.

In accordance with this invention, in this case also, water, other aqueous liquid or steam is introduced, either at a level above the contents of the vessel or, preferably, into the heating tube for the hot gases.

In the accompanying drawings: Fig. 1 is a diagrammatic section of a calcining kettle according to Patent 1 488 665 modified in accordance with this invention: Fig. 2 is a longitudinal section of a burner tube suitable for the kettle of Fig. 1 and Fig. 3 is a diagrammatic section of a calcining vessel according to Application 80 00250 modified in accordance with this invention.

As shown in Fig. 1, a calcining kettle 10 comprises a vessel 11 surrounded by a heating chamber 12, which is provided with a burner 14 or other supply of hot gas to heat the contents of the vessel indirectly by conduction through the walls, and especially through the bottom, of the vessel.

Conventional cross-flues 1 5 assist in the transfer of heat to the contents of th6 vessel. The lid 1 6 of the kettle is provided with openings 1 7 for the introduction, preferably continuously, of ground or otherwise powdered calcium sulphate dihydrate, and 1 8 for the withdrawal of exhaust gases to a dust collector. A mechanical stirrer or agitator 1 9 is driven by a shaft 20 extending through the lid.

An overflow weir or discharge tube 21, protected by a baffle 22, is provided for discharge of calcined material in accordance with British Patent 1 018 464 and a conventional valvedbottom outlet 23 is also provided.

To apply the principles of Patent 1 488 665, a generally vertical tube 24 is located within the vessel and fitted with a burner supplied with fuel and air by respective line 25 and 26. The hot combustion gases produced by the burner are introduced directly into the calcining mass by means of openings in the lower end 27 of the tube.

In accordance with the present invention, water, steam or an aqueous solution is sprayed into the calcination vessel above the mass of calcining material. Examples of positions at which a water spray may be located in a kettle are indicated at 30 in the upper region 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 above the level of the mass in the vessel 11 proper.

A preferred construction of the hot gas tube 24 and associated equipment is shown in Fig. 2. At the head of the tube, above the level 34 of the kettle lid, are provided gas and air inlets 25 and 26 respectively together with water inlets 35. A radial type burner 36 surrounded by a conical skirt 37 is fitted within the tube 24 and connected to the gas inlet. In the embodiment shown in the drawing, the burner is provided just below the level 38 of the bed of material in the vessel 11, but in other embodiments the burner may be located at a higher position, for example immediately below the level of the kettle lid 34. The bottom of the tube 24 has lateral openings 39 and the lower end of the tube is completely open.

Two hollow cone spray jets 40 are mounted within the tube 24 at about the level of the burner 36 for the introduction of moisture in accordance with the present invention. The jets are connected by water inlets 35 to a supply of suitable water under pressure.

In operation, powdered gypsum is introduced continuously through the opening 17 onto the vessel 1 1, where it is partly heated indirectly from the heating chamber 12. At the same time, fuel and air introduced into the tube 24 provide further heat by the direct introduction of hot combustion gases, which also agitate the material in the vessel. The contents of the vessel are also agitated and fluidised by the water vapour evolved during calcination, while the material in the vessel is further agitated by the stirrer 1 9. The calcined product overflows continuously from the discharge tube 21, the baffle 22 preventing direct access of possibly uncalcined material from inlet 1 7 to the outlet 21.Water or an aqueous solution is sprayed into the gases within the tube 24 by means of the spray jets 40 and has the effect of reducing to a negligible level the formation of insoluble anhydrite in the kettle.

As a further example, illustrated by Fig. 3, the invention will be described with particular reference to the production of calcined gypsum in accordance with U.K. published specification No.

2043219A.

As shown in Fig. 3, a vessel 45 of inverted conical form has a rounded bottom 46 of restricted area and carries a cylindrical upper portion 47, which in turn is ciosed by a lid 48 fitted with a feed pipe 49 for the material to be calcined, such as powdered gypsum, 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 lid 48 into the interior of the vessel.

The gypsum feed pipe 49 is provided with a metering valve in the form of a rotary feeder 52, which is connected to a gypsum bunker 53. The vessel 45 and upper portion 47 are suitably lagged as indicated at 54.

The normal level of powdered material in the vessel when it is operating is indicated at 10. An outlet for calcined material is provided in the form of an external overflow weir 55 connected to a lead-off pipe 56. A valved bottom discharge gate 57 is also provided.

The heating tube 51 extends downwardly substantially along the central vertical axis of the conical vessel 1. It is open at its lower end 58 and terminates adjacent to the bottom 46 of the vessel. The tube is also provided in the lower part of its wall with symmetrically disposed holes 61 further facilitating the distribution of hot gases into the material being calcined. The distribution of gases emerging from the tube 51 is further enhanced by the provision of an upright cone or other protrusion 62 of heat resistant material, which is seated on the bottom 46 of the vessel directly beneath the tube opening 58.

Fuel gas, for example natural gas, is supplied through a pipe 63 to a gas burner 64 of a nozzlemix type located within the tube 51 at approximately the level 10 of the material in the vessel. Air is supplied separately to this burner through an air pipe 65 from a fan 66. The hot gaseous products of combustion pass downwardly through the tube 51 and leave through its open end 58 and the holes 61. An auxiliary air supply to the heating tube is not generally used with this system.

In accordance with the present invention, water or an aqueous solution, steam or other form of moisture is preferably sprayed into the tube 51, either in the upper region of the vessel as indicated at 30, or lower in the tube, for example near the level of material 10 or in the vicinity of the burner 64, as indicated at 31. Alternatively or additionally, moisture can be sprayed into the free space in the vessel in the vicinity of the exhaust gas outlet 50 or at least partially into the outlet pipe itself, as indicated at 32.

Claims (14)

1. A process of calcining gypsum in which a bed of powdered gypsum is heated directly by the introduction of hot gas into the bed to maintain it at a temperature appropriate to the production of calsium sulphate hemihydrate, characterised in that moisture is sprayed into the gaseous phase and the formation of insoluble anhydrite is thereby reduced.

2. A process according to claim 1 wherein the moisture is sprayed into the hot gas prior to contact of the latter with the gypsum.

3. A process according to claim 1 wherein the moisture is sprayed into a region above the gypsum bead occupied or traversed by exhaust gases from the bed.

4. A process according to claim 2 wherein the hot gas is introduced into the gypsum in a vessel through a tube extending generally downwardly from the top of the vessel and opening in the lower region of the vessel, and the moisture is sprayed within such tube.

5. A process according to claim 4 wherein the hot gas is supplied by a burner mounted within the tube, and the moisture is sprayed at substantially the same level as the burner.

6. A process according to any of claims 2 to 5 wherein the gypsum is contained in a vessel and is additionally heated indirectly by thermal conduction through the walls of the vessel.

7. A process according to claims 4 or 5 wherein the gypsum is contained in a vessel having a lower region which is shaped to direct the gypsum towards the vicinity of the lower end of the tube whereby hot gas from the tube agitates and heats all the material in the vessel.

8. A process according to claim 6 or 7 wherein the gypsum is introduced continuously into the vessel and overflows or is withdrawn continuously therefrom in a calcined condition.

9. Apparatus for calcining gypsum comprising a calcination vessel for containing a bed of gypsum to be calcined and means for direct introduction of hot gas into the bed of gypsum, characterized by means for spraying moisture into the gaseous phase in or adjacent to such vessel.

10. Apparatus according to claim 9 wherein the vessel is provided with a tube for the introduction of hot gas extending generally downwardly within the vessel and opening in the lower region thereof, and at least one spray jet for moisture is located within the tube.

11. Apparatus according to claim 10 wherein a burner is fitted within the tube to provide a source of hot gas, and at least one water spray is fitted within the tube at about the level of the burner.

12. Apparatus according to claim 9, 10 or 11 wherein the vessel has thermally conducting walls and is surrounded by a heating chamber for indirect heating of the contents of the vessel.

1 3. Apparatus according to claim 9, 10 or 11 wherein the vessel has a lower region shaped to direct the contents of the vessel in operation towards the lower end of the tube.

14. Apparatus according to any of claims 9 to 1 3 provided with means for continuously introducing powdered gypsum and a discharge conduit or weir by which calcined material is continuously withdrawn.

1 5. Apparatus according to claim 9 wherein at least one spray jet for moisture is mounted in or adjacent to the vessel at a level which is above the bed of gypsum during operation, in a region which is occupied or traversed by exhaust gases from the bed.