IMPROVEMENTS IN METHOD AND APPARATUS FOR CALCINATION

Abstract: In a method an apparatus for the heat treatment of gypsum or like material, in which the material is heated in a vessel (2) by indirect heat exchange with hot gas product by the fluidised combustion of a fuel, material is fed into a first fluidised bed (A) of the material. Partially treated material is removed from the lower region of the first bed (A) and introduced with the assistance of compressed air into the upper region of a second fluidised bed (B). Treated material is removed from the second bed (B). The beds (A and B) of material are both fluidised by means of the introduction of the hot gas product of the fluidised combustion bed, further heating the beds (A and B). The first bed (A) may be operated at a lower temperature than the second bed (B), although both beds may receive substantially the same heat input.

Description: The present invention relates to a method and apparatus for calcining calcium sulphate dihydrate or gypsum, and similarly heat-reactive minerals, by establishing at least one fluidisable bed of the material, heating the material in the bed by indirect heat exchange with a hot gas, fluidising the material in the bed by introduction of the same hot gas, and removing treated material from the bed. Such a method and apparatus for calcining gypsum is described in out GB-A- 2 027 859, in which one or more individual beds of gypsum are heated by indirect heat exchange with burning fuel in a fluidised bed combustion unit and the gypsum is simultaneously fluidised and directly heated by the resulting combustion gases, fresh material being fed separately into the or each bed and calcined product being withdrawn from the or each bed. We have found that hemihydrate obtained by this single stage process requires the use of substantial quantities of a retarding agent to adjust the setting time to a convenient value for building plaster formulations. In addition, we have found that some of the gypsum fed to the treatment bed of such a single stage calciner tends to pass from the inlet directly to the outlet with the result that a significant proportion of the gypsum leaves the calciner untreated and persists as dihydrate in the product. Paradoxically, some material tends to be retained in the bed for prolonged periods and this leads to the formation of soluble anhydrous calcium sulphate which again emerges in the product. Both of these by products reduce the quality of the stucco produced. According to a first aspect of the present invention there is now provided an improved method of calcining calcium sulphate dihydrate or other heat-reactive material which is characterised in that untreated material is introduced into a first bed, partly calcined material is transferred from the first to a second or subsequent bed, and calcined product is removed from the last of the beds, hot gas being introduced into each bed for fluidisation after having transferred some of its heat by indirect heat-exchange to the beds in which calcination takes place. The hot gas may be produced by fluidised bed combustion of a solid fuel, for example coal, or a liquid fuel, for example heavy oil, and the entire fluidised bed combustion unit is preferably in indirect heat exchange relationship with the beds of material. The overall rate of the calcination reaction is reduced in comparison with the single stage calcination process. We have found that the resulting product requires less retarder to be used, as compared with the product of the single stage process, to achieve the setting time required for an acceptable plaster formulation. In addition, the multi-stage process results in a more even heat-treatment of the gypsum and reduces the proportions of dihydrate and of soluble anhydrite in the product. The efficiency of the calcination process is, therefore, increased and the quality of the product improved. The product of the invention has properties suitable for the manufacture of plasterboard. According to a second aspect of the present invention there is provided calcination apparatus which comprises at least two containers for respective beds of material to be calcined, means for feeding untreated material into a first container, means for removing calcined product from the final container, ducting for hot gas in indirect heat exchange relationship with the containers, and means for introducing the hot gas from the ducting into the lower regions of the containers for fluidisation of the beds therein, and is characterised by means for transferring material from the first container to the second container. GB-A-1 549 029 describes a multi-stage calciner in which gypsum is passed successively through a series of beds, being calcined in a first group of the beds, which are heated by a fluid circulating between indirect heat exchangers in the beds and heating apparatus in the form of an exchanger or boiler. Combustion air for the latter is preheated by indirect heat exchangers in a second group of beds in which the calcined product from the first group is cooled by cold air introduced to the exchangers therein. The preheated air is also used for fluidising the beds of both groups. In the grouping and functions of the beds and the origin and thermal history of the gas used for fluidisation, this method and apparatus differs radically from those of this invention. The invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a vertical calcination apparatus in accordance with the invention; Figure 2 is a diagrammatic view of an arrangement in plan of an apparatus incorporating the features of Figure 1. The calcination apparatus shown in Figure 1 comprises a steel vessel 2 having a rectangular base 4, two side walls 6 and two end walls not shown in the drawing. The vessel is provided with a sloping roof 8, extending from the upper edge of the side walls 6. At the apex of the roof is an outlet 10 for exhaust gases from which an exhaust duct 12 extends upwardly. An inlet 14 in one of the side walls 6 is provided for the entry of powdered gypsum to be calcined, which is supplied by means of a conventional screw feed 16. An outlet weir 18 for the calcined product material is provided in the opposite side wall. A baffle plate 20 spaced from the side wall at the outlet extends vertically downwards within the vessel 2 to a level near the base 4. This ensures that material reaching the outlet weir is drawn from the lower region of the vessel 2. The apparatus further comprises a combustion chamber 22, formed of two thermally conductive heat-exchange side walls 24, two end walls (not shown in Figure 1), and a horizontal roof 26. The combustion chamber is disposed symmetrically in the centre of the vessel 2 and, together with partition walls (not shown in Figure 1) that extend from the chamber 22 to the end walls of the vessel 2, divides the latter into two compartments A and B, which serve as containers for the beds of material being calcined. The lower portions of the side walls 24 extend downwards beyond the base 4 and are sealed to the wall of a horizontal gas supply tube 28 arranged longitudinally beneath the vessel 2. The upper portion of the wall of the gap supply tube is provided with a plurality of stand pipes 30 to allow gas from the gas feed tube 28 to enter the combustion chamber 22. A feed pipe 32 for coal extends vertically downwards through the roof 8 of the vessel 2 and through the combustion chamber roof 26 into the combustion chamber 22. A coal feed control valve 34 in the feed pipe 32 enables the apparatus to be isolated from the coal supply. Horizontal ducts 38 for hot combustion gases extend outwards from the upper portion of the combustion chamber 22 and through the vessel sidewalls 6. These ducts communicate with vertical ducts 40 extending downwards to respective plenum chambers 42 extending alongside the lower portions of the vessel side walls 6. A plurality of perforated sparge pipes 46 extend horizontally from the plenum chamber 42 into the respective lower regions of the compartments A and B of the vessel 2. One arrangement of the features shown in Figure 1 is illustrated diagrammatically by the plan of Figure 2, in which those features are indicated by the same reference numerals. Some of the fittings are omitted for the sake of simplicity. The two compartments A and B of the vessel 2 completely surround the combustion chamber 22, for efficient heat exchange, and are separated by partition walls 50. Each partition wall has a weir opening 54 at a height corresponding to the height of the outlet 18 to allow material from the compartment A to pass into compartment B. A vertical baffle 56 is arranged on the side of each weir in compartment A to form a duct having an inlet in the lower region of comparment A and communicating by way of the respective weir with compartment B. A compressed air feed pipe 58 extends upwards through the base of the vessel 2 into each such duct, through which compressed air can be introduced to assist the flow of material from the lower region of the compartment A up the respective duct and over the weir into comparment B. In the arrangement shown in Figure 2 gypsum is supplied to the screw feed 16 from a hopper 60 by means of a mechanical feed system 62 which operates in response to a controller 64 supplied with data from a thermocouple 66 in compartment B. To assist in initial loading of the calcination apparatus gypsum powder it is advantageous to have, in addition to the screw feeder 16, a second screw feeder 68 (Figure 2) arranged to feed gypsum directly into the compartment B. A diverter 70 may be included to control the supply of gypsum to the feeders 16 and 60. Once continuous operation is established, gypsum is fed only to the bed in compartment A by means of the screw feeder 16. The bed in compartment B is then maintained by the flow of material from bed A over the weirs 54 in the partition walls 50. In operation, once the beds A and B of powdered gypsum have been established, fuel is continuously fed through the fuel feed pipe 32 (Figure 1) to the fluidised combustion bed in the chamber 22. Air under pressure is supplied to the tube 28 and enters the combustion chamber 22 through the stand pipes 30 to fluidise the combustion bed. The material in the beds A and B is heated by indirect heat transfer through the walls 24 of the combustion chamber, while the hot combustion gases pass along the ducts 38 and 40 into the plenum chambers 42 and are delivered through the sparge pipes 46 into the lower regions of the beds A and B. The beds are fluidised by the hot combustion gases, assisted by the vaporised water of crystallisation evolved by the material, and the material is further heated. Exhaust gases and water vapour leave through the exhaust duct 12. In continuous operation the feed rate of the gypsum is controlled to maintain the beds A and B at the optimum operating temperatures, this being most conveniently accomplished by means of the thermocouple 66 arranged to monitor the temperature of the bed B. The signal generated by the thermocouple is processed by the controller 64 to adjust the gypsum feed rate as required. The flow rate of compressed air into the ducts 58 is adjusted to maintain a constant flow of material over the air-assisted weirs 54 from bed A and bed B and thus to maintain a constant level in bed A. The final product, for example, calcium sulphate hemihydrate, leaves the vessel 2 at the outlet weir 18. The apparatus may be conveniently arranged and operated so that the ratio of heat input to the beds A and B is between 25:75 and 75:25, and is preferably about 50:50. The beds A and B are preferably substantially equal in size. In practice, the bed A operates at a lower temperature than the bed B. Typically, the bed A operates in the range 120°C to 135°C and bed B in the range 140°C to 160°C. Preferably, bed A is operated at about 122°C and bed B at 148 to 150°C. The overall rate of the calcination reaction is reduced in the two stage process, as compared with a single stage calcination process, but this is achieved without necessitating a reduction in the total output of the calciner. The product of two stage calcination contains lower levels of soluble anhydrite and residual gypsum than the single stage product. An improved quality stucco can, therefore, be achieved from two stage calcination. The application of the process and apparatus to the calcination of gypsum is one example of the application of this invention. It will be understood that the process and apparatus can be applied to the calcination of other heat-reactive materials.

Claims: A method of calcining calcium sulphate dihydrate or other heat-reactive material which comprises establishing at least one fluidisable bed of the material, heating the material in the bed by indirect heat exchange with a hot gas, fluidising the material in the bed by introduction of the same hot gas, and removing treated material from the bed, characterised in that untreated material is introduced into a first bed, partly calcined material is transferred from the first to a second or subsequent bed, and calcined product is removed from the last of the beds, hot gas being introduced into each bed for fluidisation after having transferred some of its heat by indirect heat-exchange to the beds in which calcination takes place. A method according to claim 1, wherein the hot gas is produced by fluidised bed combustion of a fuel in indirect heat exchange with the beds. A method according to claim 1 or 2, wherein the first bed operates at a lower temperature than the second or subsequent bed. A method according to claim 3, wherein the beds comprise calcium sulphate and the first bed operates in the temperature range 120 to 135°C, preferably about 122°C. A method according to claim 4, wherein the second bed operates in the temperature range 140 to 160°C, preferably at 148 to 150°C. A method according to any preceding claim, wherein the respective beds receive substantially equal heat input. A method according to any preceding claim, wherein partially calcined material is removed from the lower region of a first bed and introduced into the upper region of a second bed. A method according to claim 7, wherein the movement of partially calcined material between the first bed and the second bed is assisted by a current of gas. Apparatus for calcining calcium sulphate dihydrate or other heat-reactive material, which comprises at least two containers (A,B) for respective beds of material, means (16) for feeding untreated material into a first container (A), means (18) for removing calcined product from the final container (B), a supply (22) of hot gas, ducting (24,38) for bringing the hot gas into indirect heat exchange relationship with the containers, and means (40-46) for introducing the hot gas from the ducting into the lower regions of the containers for fluidisation of the beds therein, characterised by means (54) for transferring partly calcined material from the first container to a second container. Apparatus according to claim 9, wherein the supply of hot gas is a fluidised bed combustion unit (28-32) for a fuel which is in indirect heat exchange relationship with the container (A,B). Apparatus according to claim 10, wherein containers (A,B,) are immediately adjacent to the combustion unit (28-32) and share heat conduction walls (24) therewith. Apparatus according to claim 9, 10 or 11, wherein the means for transferring material from the first container (A) to the second container (B) comprises a duct (56) having an inlet communicating with a lower region of the first container and an outlet in an upper region of the second container. Apparatus according to claim 12, further comprising means (58) for introducing a flow of gas into the duct (56) for assisting the movement of material from the first container (A) to the second container (B).

Inventors: WATKINS, DAVID WILLIAMS
Applicants: BPB INDUSTRIES PUBLIC LIMITED COMPANY
Application year: 1989
Main classification: C04B11
IPC codes: B01J6, C04B11, B01J8