Note: Descriptions are shown in the official language in which they were submitted.
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This invention relates to an improvement in the method
and apparatus for calcining calcium sulphate dihydrate or gypsum.
In calcining the dihydrate tohemihydrate, any tendency
to produce insoluble anhydrite at the expense of hemihydrate is
disadvantageous since in certain applications, this will reduce the
effect 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 with-
out 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.
Thus, in the process of calcining gypsum in which a bed
of powdered gypsum in a vessel is heated directly by the introduction
of hot gas through a tube extending into the bed and regulation of
the hot gas to maintain the bed at a temperature appropriate to the
production of calcium sulphate hemihydrate, this invention provides
the improvement which comprises spraying moisture into the gaseous
phase in or above the bed whereby the formation of insoluble
anhydrite is reduced.
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Moreover, in an apparatus for calcining gypsum which
comprises a calcination vessel for containing a bed of gypsum to
be calcin~d and a tube extending into said vessel for direct
introduction of hot gas into direct contact with said bed of
gypsum, this invention also provides the improvement which comprises
moisture spray means disposed in said tube or vessel.
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 2043219A. Whilst this detailed description of the
invention relates to these two methods and apparatus, it should be
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~mderstood that the invention can be applied to other methods and
apparatus for continuously calcining calcium sulphate, and more
especially to equipment which calcines calcium sulphate dihydrate
in a rapid manner by direct heating.
S 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 non-reactive hot gases, preferably hot
gaseous combustion products, into the interior of the mass of the
dihydrate within the vessel through a tube extending generally down-
wardly 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 thls 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 thoughput 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 gas or gaseous combust-
ion products can be used, it is at present preferred to use a fuel
burner iocated 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 intc 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
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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 Published United Kingdom Specification Number
2 043 219A describes a method and apparatus for heat-treatment of
lo 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 associ-
ated with conventional kettles and to thermally lag the system, so
increasing the thermal efficiency and reducing capital and mainten-
ance costs.
According to that application the method comprises intro-
ducing 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 lo~est region, irtroducing 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 heating extends
from the bottom region through substantially all the material in the
vessel, and withd,dwing heat-treated 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
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wall heating ~ay 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. l; and
Fig. 3 is a diagrammatic section of a calcining vessel
accordlng to U.K. published specificatlon No. 2043219A 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 15 assist in
the transfer of heat to the contents of the vessel. The lid 16 of
the kettle is provided with openings 17 for the introduction,
preferably continuously, of ground or otherwise powdered calcium
sulphate dihydrate, and 18 for the withdrawal of exhaust gases to
a dust collector. A mechanical stirrer or agitator 19 is driven
by a shaft 20 extending through the lid. An overflow weir or dis-
charge tube 21, protected by a baffle 22, is provided for discharge
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~f calcined material in accordance with British Patent 1 018 464
and a conventional valved-bottom 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 materials. Examples of positions at which a
water spray may be located in a kettle are indicated at 30 in the
u~per 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 associ-
ated 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 ~'J and the lower end of the tube is completely open.
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T~o 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 11, where it is partly
heated indirectly from the heating chamber 12. At the same time,
fuel and air introduced into the tune 24 provide further heat by
the direct introduction of hot combustion gases, which also agitate
the material in the vessel. m e 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
19. The calcined product overflows continuously from the discharge
tube 21, the baf1e 22 preventing direct access of possibly uncalcined
material from inlet 17 to thé 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 refcrence to the production of
calcined gypsum in accordance with U.X. published specification
No. 2043219A.
As shown in Fig. 3, a vessel 45 of inv3rted conical form
has a rounded bottom 46 of restricted area and carries a cylindrical
upper portion 47, which in turn is closed by ~ 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 connçcted to a dust
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collector (not shown~. ~ heating tube 51 which is described in more
detail below, also passes through the lid 48 into the interior of
the vessel. m e 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. m e vessel 45 and upper portion 47 are suitably
lagged as indicated at 54.
m e normal level of powdered material in the vessel when
it is operating is indicated at 10. An outlet for calcined material
is provided in ~he form of an external overflow weir 55 connected to
a lead-off pipe 56. A valved bottom discharge gate 57 is also
provided.
Ihe heating tube Sl extends downwardly substantially along
the central vertical axis of the conical vessel 1. It is open at
lts 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 thé materiai being calcined. m e
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 nozzle-mix type located within the
tube 51 at approximately the level 10 of the ma'erial in the vessel.
Air is supplied separately to this burner through an air pipe 65
from a fan 66. The hot gaseous products of co~ustion pass down-
wardly 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
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~enerally 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.
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