GB2213810A - Calcium-sulphate - Google Patents

Description

23K 53 700 2 2- 17, 8 10 Process and apparatus for producing calcium

sulphate hemihydrate in the alphaconfiguration

Description

The invention relates to a process and apparatus that may be used for producing calcium sulphate hemihydrate in the alpha-configuration.

The alpha-hemihydrate form may be formed from calcium sulphate dihydrate by wet dehydration in a moist atmosphere under pressure at a little above 1OCC, or in certain acid solutions or salt solutions without pressure and at lower temperatures. This invention relates to the production of the alpha-hemihydrate form of calcium sulphate by the first-mentioned process above. In this case, the dehydration takes place in an autoclave and, conventionally, gypsum rocks of 30 to 50 mm in diameter for exainple, are heated at a steam pressure of 2 to 4 bars for about 3 to 4 hours.

Special problems arise in the dehydration of synthetic gypsums, in particular gypsums from chemical processes and gypsums from flue gas desulphurisation, because these have much finer particles than natural gypsums and they can have particle sizes of markedly below 100fAm. The crystal habit also differs from that of natural gypsums. Furthermore, the absorbed bound surface moisture content of these gypsums is, at 10% or more by weight, markedly higher than that of natural gypsums (about 1 to 3% by weight).

Flue gas gypsums in particular arise in large quantities as a result of the statutorial flue gas desulphurisation units, and there is an urgent demand for putting these gypsums to a worthwhile use.

The-dehydration of fine-particuled industrial gypsums to give the alphahemihydrate form is conventionally carried out by first preparing of water, suspensions, sometimes of more than 50% by weight of water, and then dehydrating these in an autoclave under the action of pressurised steam in water-saturated surroundings. This process requires very expensive equipment because the suspensions must be agitated during their conversion in the autoclave and subsequently dehydrated by means of dehydrating equipment (for example filter presses), before they are dried for removing the adsorptively-bound residual water.

Also, the problems of rehydration between the autoclave and the dryer have not yet been satisfactorily solved. Further, in the conventional dehydration of synthetic gypsums there is unavoidable slurrying which therefore entails at least an increase in energy consumption in the subsequent dehydration. In fact, it is then necessary not only to heat the solids content (of a comparatively low specific heat capacity) to the reaction temperature, but also the considerable proportion of water in the suspension, so that only a fraction of he input energy is utilised for the actual dehydration.

Accordingly, there is still a need to provide a process and apparatus suitable for converting gypsums into their alpha-hemihydrate states at low cost, quasi-continuously, and substantially avoiding rehydration. Such a process should be applicable to both natural and synthetic gypsums, (including industrial gypsums), especially gypsums derived from flue-gas desulphurisation. Further, it is possible in the dehydration of synthetic gypsums to avoid slurrying of the gypsums.

We have now been able to devise a process which we believe meets the above requirements, and according to bhe aspect of the invention, we provide a process for the preparation of calcium sulphate in the form of the alpha-hemihydrate, which comprises: 10 a) b) c) d) e) f) transferring an open vessel containing calcium suphate dihydrate into an autoclave; autoclaving the material with pressurised steam until it has been converted to calcium sulphate hemihydrate in the alpha-configuration; removing the vessel from the autoclave and discharging the moist material directly into a heated trough; transferring the material from the trough to a heated conveyor and conveying it to a dryer; freeing moist material in the dryer from any further adsorped residual water; removing the dried material.

Holes or elongate channels are preferably punched from above into the starting material in the open vessel prior to autoclaving. As a result, a larger free surface area of the material becomes available for hemihydration in the autoclave. The diameter of the channels should desirably here be between 0.5 and 5 cm, preferably between 1 and 2 cm. The channels can equally be produced in the material by drilling, turning or in other ways.

moreover, it is advantageous that the autoclave be completely deaerated after it has received the vessel, and is only then charged with 4 pressurised steam. The temperature in the autoclave is preferably then adjusted to between 105 and 180C, and the hydrothermal conditions in the autoclave are then maintained until the calcium sulphate dihydrate has been converted to calcium sulphate hemihydrate in the alphaconfiguration, before the'ressure in the autoclave is lowered again to normal pressure and the calcium sulphate hemihydrate formed is removed and transferred into the heated trough.

100% conversion will as a rule not be achievable here. The term "complete conversion" is therefore to be understood in such a way that it means a conversion which is as substantial as possible, for example 98% and higher.

In this process, it is not necessary for the starting material employed to be an aqueous suspension, which has a catalytic activity with respect to the crystallisation of alpha-hemihydrate, as a medium for the dehydration reaction. Rather, it has been discovered that the air present between the particles is an important problem during the dehydration in the autoclave. In fact. the heat transfer during the subsequent autoclave treatment is made considerably more difficult by sInall or large air pockets between the individual solid particles of the starting material. Depending on the form of the charged material in the autoclave, temperature distribution irregularities in the autoclave still occur even during prolonged autoclave treatment, thus causing differing degrees of dehydration and hence varying qualities of the end product.

By contrast, this invention makes it possible to eliminate air pockets between the solid particles.

1 This is helped by the punched-in holes or channels, through which the air can be more readily extracted during the deaeration. There remains a kind of "skeleton" of the solid particles with cavities located inbetween, which can then be penetrated easily and uniformly by the pressurised steam subsequently fed.into the autoclave.

Heating of the starting material to the temperature required occurs with the heat which is released during the condensation of the steam onto the colder surfaces of the solid particles.

As a result of the "open skeleton", not only will the outer surface of the charge directly facing the steam very rapidly assume the particular temperature in the autoclave, but also the particles in the interior of the charged material which the steam can reach rapidly and directly via the deaerated cavities (channels) in the material.

The result of this is not only a much more rapid, but also much more uniform heat transfer to the material, so that the dehydration then taking place can proceed within a much shorter time and in a more uniform manner.

There are various methods possible for te deaeration procedure. one possibility is to flush the autoclave with steam. The steam then forces out the air between the individual particles of the charged material; any holes or channels which have been pressed in having a positive effect. This flushing with steam can be carried out at atmospheric pressure.

An alternative method provides for the deaeration to be carried out by means of a device for gas evacuation, for example a vacuum pump. Here again, the holes or channels are, in a manner of speaking, deaeration channels, along which air can be extracted.

The effect is the same, in that the air between the solid particles is completely extracted.

In a third method, the procedure is such that the autoclave is first charged with steam to a pressure of about 2 to 3 bars, or higher if desired. The air between the particles is thus compressed and initially cannot escape. By subsequent letdown (lowering of the pressure), it is possible to pull out the air between the particles and, at the same time, to remove it via any holes or channels which have been punched in. The greater the lowering of the pressure (at most down to atmospheric pressure), the more complete is the deaeration which takes place. Experiments have shown that when a pressure of 2.5 bars is set in the first stage, a subsequent let-down to about 1.5 bars is sufficient to achieve almost complete deaeration.

Experiments have further shown that it is as a rule sufficient to take the evacuation to about 70%, in order to extract such a quantity of air that an adequate pore space is available ' thereafter for the subsequent charged pressurised steam to be completely and uniformly introduced into the cavities.

In order to prevent subsequent rehydration of the material removed from the autoclave, in a further embodiment of the invention the material is held at a temperature above 800C during the period between removal from the autoclave and transfer into the dryer.

1 This is preferably effected by means of indirect heat transfer, using apparatus sections which will be described below in more detail.

At the same time, it is advantageous here to comminute the partly agglomerated material in order to produce a str6hm of fine material between the autoclave and the dryer. In this way, the proportion of free surface area is significantly increased and the dryer can thus operate more effectively. As already stated, it is proposed according to the invention, to heat both the trough, into which the material. is charged from the vessel, and the conveyor which takes the material to the dryer.

For this purpose, it is also proposed to construct the trough preferably with double walls and to heat the cavity in the wall by means of a heating medium, for example hot steam or heat transfer oil. Equally well, the trough can also be heated electrically. - The trough, in apparatus according to the invention, fulfils a kind of holding function. It should discharge the material received from the vessel as continuously as possible to the conveyor. In a further embodiment of the invention, the trough is fitted with a grating between its charging end and discharging end by means of which the stream of material is again loosened. In place of the grating, it is possible to stretch tension ropes transversely over the cross-section of the trough, which is preferably funnel-shaped.

This invention provides in a further embodiment, that the conveyor is a troughed-chain conveyor which is particularly suitable for conveying the hot, moist and tacky calcium sulphate hemihydrate continuously and without formation of lumps.

The troughed-chain conveyor is in turn also heated, prefdrably analogously to the heating of the trough, via housing constructed with double walls, or electrically, as described above.

The housing is constructed so.as to be completely closed, particularly in transition either to or from the trough, in order to prevent any heat losses and to improve the safety of the installation in use.

The troughed-chain conveyor is preferably constructed with an upward slope towards the dryer, which promotes the division of the stream of material along the transport route.

For the drying installation, the irivention proposes to use a paddle dryer, through the paddles, shafts and/or shell of which can flow a heat transfer medium. The hollow blades may be attached via ducts to the hollow shaft, which will allow the heat transfer medium to pass along the shaft and circulate within the paddles.

Paddle dryers have proved suitable for the thermal treatment of, for example, oil seeds, tobaccos or cereal flour, but they can surprisingly be employed equally advantageously for drying calcium sulphate hemihydrate. Good flow through of material here assures uniform drying and product quality.

Heat transfer to the material is by direct contact 1 1 j with the heated walls of the individual components of the dryer. Large quantities of air for transferring the heat are not necessary, and only a small air stream (or gas stream) is required for removing 5 the 1apours.

The'15addle dryer consists of a trough with an enclosed shell, enclosing for example, two or four driven shafts. Hollow, wedge-shaped paddles are fitted to the axis of the shafts, perpendicular to the shaft direction and parallel next to one another. The paddles of one shaft move between the paddles of the neighbouring shaft. This produces a high surface/volume ratio.

The calcium sulphate hemihydrate is fed in on one side of the trough from the conveyor and is transported by hydrostatic forces to the other side. The discharge is via an appropriate overflow.

Accordingly, the paddles do not serve for transporting the material, but only for heat transfer.

The entire installation should here be operated in such a way that the treated material never cools to a temperature below about 800C between the outlet end of the autoclave and the outlet end of the dryer.

The apparatus according to the invention can be operated particularly advantageously when the pressurised steam taken out from the autoclave is at the same time used for heating the downstream equipment sections.

The loosening of the starting material, as described at the outset for the subsequent deaeration, can be promoted if the vessel is perforated on the wall sides and/or bottom side, so that any excess water in the material can escape before entry to the autoclave and especially during the punching of the holes and/or channels. For this purpose, the Vessel is preferably arranged via appropriate spacers on a tray, into which the water can flow.

Th - e punching device itself can be of simple construction. For example it can comprise a plate with cylindrical pins which protrude from the underside and are pressed into the starting material.

Furthermore, in order to encourage the achievement of a quasi-continuous transport of material as far as possible, a further embodiment of invention is a tipper device for the transport vessel, by means of which the material is transferred from the transport vessel into the cylindrical trough. The trough is preferably constructed with a cover which is closed as soon as the vessel has been completely emptied. in this way, heat loss of the material from the autoclave to the dryer is even further prevented.

By means of the process according to the invention and the corresponding apparatus, quasi-continuous production of calcium sulphate hemihydra ' te in the alpha-configuration is made possible even for the dehydration of industrial gypsums, and the energy consumption required for the dehydration is extremely small, since the starting material does not have to be slurried first to give suspensions, but can be dehydrated directly.

A high throughput rate is achieved by the use of large-capacity vessels which can take, for example, 1,500 kg of gypsum or more.

8 S The invention is further explained below in more detail with reference to the accompanying drawings which shows, by way of example, apparatus according to the invention in a diagrammatic way.

In the drawings:

Fig. 1 is a diagrammatic plan of apparatus which may be used in the process of the invention; Fig. 2 is a perspective view.of a perforated vessel used in an embodiment of the invention; Fig. 3 is a cross-sectional view of a paddle dryer shaft and blade used in the apparatus.

A transport vessel 10 for receiving gypsum, in particular synthetic gypsum is shown. The transport vessel 10 consists of four perforated walls and a perforated bottom, and it is inserted via spacers (not shown) into a tray 12. Such a vessel itself is shown in Figure 2. The transport vessel 10 with the tray 12 is arranged to be movable on four wheels. The capacity of the transport vessel 10 is typically lm 3 The tray 12 serves to receive any water yhich emerges through the perforations of the transport vessel 10.

The transport vessel 10, once filled with gypsum, is situated underneath a punching unit 14. The latter consists of a plate 14a and a plurality of cylindrical rods 14b protruding vertically downwards from the plate. The punching unit 14 is vertically movable by means of a lifting device (not shown), and in particular it is moveable into the mass of gypsum present in the transport vessel 10.

11.

When the punching unit 14 is moved down and the rods 14b enter the gypsum, elongate channels are thus created in the mass of gypsum, which remain after the punching unit has been removed, since especially industrial waste gypsum is very largely a material which remains static even after deformation.

The transport vessel 10 and the punched material is then moved into an autoclave 16, doors 18 of which are then closed.

The autoclave 16 is fitted with a device for evacuation of gases 22, the essential component of which is a vacuum pump 20. By means of the vacuum pump 20, evacuation of the autoclave 16 is then carried out, and in particular until at least 70% of the air previously present in the autoclave has been extracted.

The air present between the solid particles of the gypsum is thus sucked out and an "open skeleton" is formed.

When the autoclave is then subjected to pressurised steam in a saturated steam atmosphere, the temperature in the autoclave being adjusted to between 105 and 180C, not only will the outer surface of the gypsum very rapidly assume the particular temperature in the autoclave. but the particles, which are in the interior of the dumped charge and which can be reached rapidly and directly by the steam via the punched channels in the dumped charge and via the deaerated cavities, will also.

The moist atmosphere under pressure at the said temperatures is maintained until the conversion of dehydrate to alpha-hemihydrate has taken place f 13 - as fully as possible. As mentioned above, 100% conversion will as a rule not be achievable here. The term "complete conversionn is therefore to be understood in such a way that it means a conversion which is as substantial as possible, for example 98% and higher.

in an autoclave which is charged with, for example, three transport vessels 10 each carrying 1.5 m 3 of waste material (gypsum from flue gas desulphuris- ation), the holding time is about 1.5 hours.

The atmosphere in the autoclave is then let down to normal pressure.

Subsequently, the doors 18 of the autoclave 16 are opened again by moving them Up, and the transport vessel 10 (or vessels) is moved out of the autoclave and the material is tipped directly (arrow K) via a tipper device (not shown) into a trough 24, a cover 26 of which had been opened beforehand. The total vessel volume thus drops'into the trough 24 and the material is loosened by means of passage through a grating 30 on its way to an outlet 28 on the underside of the trough 24. Meanwhile the cover 26 has been closed again.

The trough 24 is constructed with double walls, and the hollow space between the walls is continuously charged with hot steam in order to maintain a temperature of more than WC, preferably between 90 and 1OCC, in the trough 24.

The material charged to the trough 24 thein drops sectionally onto a troughed-chain conveyor 32 which has an upward slope. The troughed-chain conveyor 32 has a completely closed housing which - like the housing of the trough 24 - is again charged with hot steam in order to prevent cooling of the transported material on its way to a dryer 36. This also prevents rehydration of the material.

The upper outlet end 34 of the troughed-chain conveyor 32 et)nnects directly into a receiving opening of the paddle dryer 36. The dryer 36 is constructed with a slight downward slope in the direction of an outlet 38, so that the stream of material is automatically transported forward on its way to the outlet 38 of the dryer 36.

The paddle dryer 36 consists of two driven shafts 42 which are arranged parallel to one another within an enclosure 46 and on which hollow, wedgeshaped paddles 40 are mounted in spaced relationship to each other perpendicular to the axis of the shaft 42, the paddles 40 on the individual shafts 42 being arranged with a slight mutual offset. Thus, the paddles 40 of one shaft 42 move exactly between two paddles 40 of the adjacent shaft (not shown). This produces a high surface/volume ratio. On the way to the outlet 38, the product is transported under the action of the hydrostatic forces. The discharge takes place via the overflow 38.

The heat transfer medium is steam which is fed into the shafts 42, and paddle blades 40 and/or the shell of the dryer 36 which are constructed as hollow bodies. The heating medium in one embodiment flows from one end of the shaft 42 to the other.

A cross-section of a paddle 40 and the path of the hot steam (arrow H) within the hollow shaft 42 are shown in Figure 3. The paddle blades 40 are attached to the hollow shaft 42 and steam passing along the shaft circulates within the paddles via R t - is - ducts 44.

As a result of the fact that, in the paddle dryer 36, the individual wedge-paddles do not themselves transport the product, but the transport takes place only under the action of the hydrostatic forces, the individual paddles only effect a loosening of the material which is in part still tacky and moist, so that the overall dryer volume is increased. Because the heat transfer to the material takes place by direct contact with the heated metal of the components of the Daddle dryer, large quantities of air for heat transfer are not necessary, and the energy consumption can accordingly be kept small.

Conversely, the apparatus can be run at high k (heat transmission coefficient) values, which improves the economics.

Because of the high dryer output, the apparatus itself can be built in a relatively small size, without having to accept capacity losses.

The wedge-shaped paddles are self-cleaning and are moved at a relatively low speed of rotation.

For a dryer having a throughput rate of about 15,000 kg/hour of calcium sulphate hemihydrate in the alpha-configuration of a moisture content of about 25% by weight at the inlet and a moisture content of approximately zero % by weight at the outlet 38, apparatus of an overall length of only 6 m is required.

The hot steam for heating the apparatus is adjusted such that the material is always at a temperature between about 100 and 130'C.

By means of the apparatus according to the invention, calcium sulphate dihydrate can be converted quasicontinuously into hemihydrate in the alpha-configuration, in particular while avoiding intermediate storLge, and considerably preventing at the same time rehydration of the material.

An alpha-hemihydrate of optimum quality can thus be taken off continuously at outlet 38 of the equipment and transferred into suitable transport vessels 10 or silos.

h C)

Claims (22)

Claims:

1. A process for producing calcium sulphate hemihydrate in the alphaconfiguration, which comprises:

a) transferring an open vessel containing calcium suphate dihydrate into an autoclave; autoclaving the material with pressurised steam until it has been converted to calcium sulphate hemihydrate in the alphaconfiguration; removing the vessel from the autoclave and discharging the, moist material directly into a heated trough; d) transferring the material from the trough to a heated conveyor and conveying it to a dryer; e) freeing moist material in the dryer from any further adsorbed residual water; and f) removing the dried material.

' - b) c)

2. A process as claimed in claim 1, wherein holes or channels are punched or drilled into the calcium sulphate dihydrate material from above before transfer into the autoclave.

3. A process as claimed in claim 2, wherein the channels are of a mean diameter of between 0.5 and 5 cm.

4. A process as claimed in any of claims 1 to 3, wherein the material is held at a temperature above 80C between its removal from the autoclave and it transfer to the dryer.

5. A process as claimed in any of claims 1 to 4, wherein the stream of material is transported in a water-tight and dust-tight apparatus between rk J discharge into the trough and transfer to the dryer.

6. A process as claimed in any of claims 1 to 5, wherein after the autoclave has received the material a) . b) is and has been closed, it is deaerated, charged with pressurised steam, the temperature in the autoclave being adjusted to between 105 and 180C, and c) the hydrothermal conditions in the autoclave are maintained until.the calcium sulphate dihydrate has been converted to calcium sullphate hemihydrate in the alpha-configuration, before d) the pressure in the autoclave is reduced to normal and e) the calcium sulphate hemihydrate formed is removed and discharged into the heated trough.

7. A process as claimed in any of claims 1 to 6, wherein heating of the material between the trough and the dryer is effected by indirect heat transfer.

8. A process as claimed in one of claims 1 to 7, wherein drying of the material in the.dryer is effected by indirect heat transfer.

9. A process as claimed in one of claims I to 8, wherein the stream of material is comminuted without grain crushing during its conveyance between the trough and the dryer and/or in the dryer.

10. Apparatus for carrying out a process according to any one of claims I to 9, comprising a) a vessel, the walls and/or the base of which may be perforated, -1 b) a discontinuously operating autoclave, c) an indirectly heated trough, 6) an indirectly heated conveyor for transferring a stream of material from the trough into e) a dryer, and the transition regions between the trought the conveyor and the dryer being constructed and arranged so as to be substantially water- tight and dust-tight and thermally insulated.

11. Apparatus as claimed in claim 10, wherein the walls of the trough and/or of the conveyor are double walls, through which a heating medium can flow.

12. Apparatus as claimed in either claim 10 or claim 11, wherein the dryer is a paddle dryer, the paddles, shafts and/or shell of which are constructed so that the heating medium can flow inside and through them.

13. Apparatus as claimed in any one of claims 10 to 12, wherein the trough, the conveyor and/or the dryer can be heated by means of steam or heated oil.

14. Apparatus as claimed in any one of claims 10 to 13, wherein the trough includes a grating which loosens the material between the point of charging and the outlet.

15. Apparatus as claimed in any one of claims 10 to 14, wherein the conveyor is a troughed-chain conveyor.

16. Apparatus as claimed in any one of claims to 15, wherein the conveyor and/or the dryer are inclined to the horizontal.

17. Apparatus as claimed in one of claims 10 to 16, wherein the vessel is constructed with perforated walls and/or bottom and rests on the bottom of a tray, appropriate spacers being provided between the base of the vessel and the bottom of the trav.

18. Apparatus as claimed in any of claims 10 to 17 which includes a punching device comprising a baseplate and cylindrical rods which protrude downwards from the baseTD1ate and are constructed so that thev can be moved into the material present in the vessel.

19. Apparatus as claimed in anv of claims 10 to 18 which includes a tipoer device for transfer-linip the material from the vessel into the trough.

20. Apparatus substantially hereinbefore descri':-)e(i and with reference to the accompanying drawings.

21. A process as claimed in claim 1 substantiall as hereinbefore described.

22. A process as claimed in claim 1 substantially as hereinbefore'described and with reference to the accompanying drawings.

Published 1988 at The Patent Office. State House. 66 71 High Holbcrn, London WCIR 4TP Further copies may be obtaine from The Patent Office. Sales Branch, St Ma:y Cray. Orpington. Rent BRS 3RD Printed by Multiplex techniques ltd, St Mary Cray. Ken'. Con 18-