Apparatus for the thermal conversion of gypsum

An apparatus for effecting the thermal conversion of gypsum to calcium sulfate hemihydrate includes a reactor having a series of fluidized bed compartments containing separate heat exchangers and defined by partitions each provided with an opening which is selected so that the material undergoing treatment progresses, without return movement, through the series of fluidized bed compartments. The apparatus is further constructed so as to permit recycling of fine gypsum particles from the outlet to the middle of the reactor.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to a process and apparatus for the thermal treatment 
of gypsum (CaSO.sub.4.2H.sub.2 O) by indirect heating to produce a 
hemihydrate product (CaSO.sub.4.O.sub.2 5H.sub.2 O). 
2. Description of the Prior Art 
Generally, it is known that in order to obtain a plaster which is rich in 
hemihydrate and is suitable for prefabrication, the method of calcination 
must first be carried out at a high temperature which imparts the required 
reactivity to the product; secondly, the gypsum must be homogeneous to as 
to avoid the presence of unburned material (non-dehydrated gypsum) and 
also of overburned material (or anhydrite II), sometimes caused by 
prolonged local contact with gases having a temperature and a partial 
pressure of water vapor which are incompatible with the stability of the 
hemihydrate form. Finally, the treatment must be economical and preferably 
continuous. 
Many processes are known which are partially effective in achieving the 
desired results mentioned above. In the processes of German Applications 
Nos. P 17 71502 and P 17 58566 the reaction rate of the product undergoing 
reaction is achieved by mechanical movement of a part of the calcination 
apparatus, in rotating furnaces or in mobile hearth furnaces with direct 
or indirect heating. In all cases, the low coefficient of heat exchange 
requires large sizes of apparatus and hence high investment costs; 
likewise, it is also necessary to use dust elimination systems; in 
addition, the moving parts require maintenance which is not insignificant 
expense. 
The reacting product is conveyed by means of a gas in the so-called "flash" 
calcination and pneumatic transport processes, as described in French 
Patent Application No. 2,202,251 and German Patent Applications Nos. P 22 
00,532 and P 21 52,940. These processes are flexible but have the 
disadvantage of requiring large volumes of air, which, in the absence of 
complex recycling, prevents any possibility of controlling the partial 
pressure of water vapor of the system undergoing conversion. In contrast, 
the air/plaster ratio is much lower in the process of French Application 
No. 74/22,621 of the applicants, due to an apparatus of a special 
geometric design which makes it possible to operate at extremely high 
temperatures and to achieve a uniform conversion coupled with an excellent 
thermal efficiency. 
Vertical transport of the material and calcination in a fluidized bed are 
also known, through French Patent No. 1,338,126 and its Addition No. 
87,866, but until now a high, gypsum-fed fluidized bed required a high 
rate of fluidization, causing substantial dispersions and necessitating 
high-capacity blowers; moreover, such a device suffers from the drawbacks 
of using a single homogeneous reactor, in that the feeding of the 
reactant, at the outlet, reflects a compromise between the quality of the 
product and the size of the apparatus. Furthermore, the partial pressure 
of steam can only be controlled with difficulty in these processes which 
have not undergone industrial development. 
The horizontal apparatus, in the form of a tunnel, of French Pat. No. 
1,288,836, where the material is agitated and advances by virtue of 
intermittent pulses of steam is also known. The steam/air ratio is high, 
which has the advantage of allowing a choice of temperature and the steam 
pressure. However, the heat exchange between the material and the heating 
elements or walls is only intermittent, limiting the maximum productivity 
of the apparatus. In addition, the pulsation apparatus is complex and 
costly. 
SUMMARY OF THE INVENTION 
We have now found a process for thermal conversion of gypsum to calcium 
sulfate hemihydrate having improved reactivity and mechanical 
characteristics for a plaster used in prefabrication which comprises 
providing a continuous fluidized bed of finely divided gypsum by means of 
a gas substantially comprising air, maintaining a feed rate of 
fluidization gas from between the minimum rate below which the bed will 
remain at rest and about six times this rate, a substantially horizontal 
movement of the fluidized material from one end of the fluid bed to the 
other, simultaneous with the progress of the dehydration, heating by means 
of heating elements immersed in the fluid bed, such that a temperature 
gradient between the mean temperature of each heating element and said 
fluidized material is at least about twenty degrees centigrade, removing 
the water vapor by-product of said dehydrating reaction and recovering at 
the outlet end of the fluid bed a product substantially being calcium 
sulfate hemihydrate. 
It is an important object of this invention to provide a new and novel 
process for economically producing calcium sulfate hemihydrate having 
improved properties useful in plaster prefabrications. 
It is a further object of this invention to provide a dehydration reaction 
for gypsum with efficient control of the kinetics of the reaction along a 
high coefficient of heat exchange in the fluidized bed. 
It is still another object of this invention to provide a process using 
indirect heating for the dehydration of gypsum which is more efficient in 
reactant conversion and product yield. 
DESCRIPTION OF THE PREFERRED EMBODIMENTS 
In accordance with this invention both natural gypsum and synthetic 
gypsums, such as those originating from the production of phosphoric acid 
and which are commonly called phosphogypsums, can be used as starting 
materials in the process and apparatus as herein described. The process 
comprises a thermal treatment of gypsum, in order to convert it into 
hemihydrate, by indirect heating in a fluidized bed; the invention 
provides the following combination: a continuous fluid bed of finely 
divided gypsum, a feed rate of fluidization gas which is between the 
minimum theoretical rate below which the bed remains at rest and about six 
times this speed, a movement of material from one end of the fluid bed to 
the other, as the dehydration reaction progresses, a supply of heat by 
means of heating elements immersed in the fluid bed, such that a 
temperature difference of several tons of degrees is set up between the 
mean temperature of each heating element and the fluidized material, and 
the removal of the water vapor produced by the reaction; and at the outlet 
end of the fluid bed, recovering a product which has been converted almost 
completely to hemihydrate and which has suitable properties for a plaster 
used for prefabrication. 
In implementing the process, a heated fluid is circulated, generally in 
separate heating elements, so as to set up, at the inlet of each heating 
element, a temperature which is practically constant and is between 
180.degree. and 300.degree. C. The temperature of the heated fluid is 
adjusted at the outlet of each heating element, by regulating the flow 
rate of the heated fluid, so that this outlet temperature is 30.degree. C. 
to 40.degree. C. below the inlet temperature. 
Fluidization conditions are achieved by regulating the feed rate. An 
average rate for fluidization gas is advantageously selected approaching 
the formation of fluidization and generally ranging between 5 and 15 
cm/sec. At each point in the reactor, the fluidization conditions are such 
that the temperature and the composition of the mass reacting and of the 
gases present, are practically homogeneous. 
Thus, for a given rate of operation, that is to say for conditions 
corresponding to a given flow rate of gypsum, a practically constant 
partial pressure of water vapor, in particular, is maintained at each 
point of the reactor by controlling the speed of the fluidization gas and 
the heat conditions. Generally, a partial pressure of water vapor is 
maintained between 130 and 550 mm. of mercury. 
The gypsum undergoing dehydration exhibits a uniform composition at each 
point of the reactor while the composition changes from one end of the 
reactor to the other from gypsum dihydrate, CaSO.sub.4.2H.sub.2 O, to 
hemihydrate, CaSO.sub.4. 0.5 H.sub.2 O. The total time the reactant 
remains in the reactor is generally regulated in such a manner that 
practically complete conversion to hemihydrate CaSO.sub.4. 0.5 H.sub.2 O 
is obtained at the outlet end. 
The gas used for fluidization and which is charged with water vapor is 
evacuated in the upper part. At the fluidization gas outlet any fine 
particles of gypsum, which may have been entrained, are collected and 
returned to the fluid bed. 
It has been found advantageous to provide, near the outlet, at least one 
device for cooling which cools the final heated product. Preferably, the 
cooling is effected by means of a device which makes it possible to 
utilize the heat from the heated product in another part of the 
installation, e.g., the fluidization gases or the primary furnace air of 
the fluid-heating device. 
The process of the invention is carried out by introducing the practically 
dry gypsum at the inlet of a fluidized bed reactor, which is usually of a 
horizontal form; the fluidized material is transported by any means known 
in and of themselves, such as inclining the reactor, passage between 
baffles or overflow between partitions, all assuring, moreover, better 
control of the progress of the reaction. 
In a particularly advantageous embodiment of the process of the invention, 
the transport of the material is carried out in a series of communicating 
compartments which constitute the fluidized bed, each of these 
compartments possessing a separate heating element and an exhaust method 
for the removal of the fluidization gases, so that for all intents and 
purposes the compartment behaves like a homogeneous reactor, the 
temperature and composition of the reacting material and of the gases 
present being maintained almost constant in each compartment. 
The process of the invention is preferably implemented in an apparatus such 
as the one represented schematically in the attached drawing. It includes 
a series of compartments 1, into the first of which the gypsum is 
introduced at 2 by means of a metering system, not shown but placed at 3 
and which could, for example, be a hopper and a dispenser. A fluidization 
gas is introduced at 4 which enters the reactor by means of diffusing 
plate 5. The fluidized bed is heated by a system of exchangers 6, in which 
a fluid circulates, it being possible to constitute this system of 
exchangers in another method of embodiment by plates or by coils. The 
fluid is reheated at 7 in an apparatus which can, for example, be an 
exchanger or a boiler. In the latter case, the fumes from the boiler can 
be recovered and used to particular advantage as dilution air for a 
pneumatic drier. Each fluidized compartment possesses a separate exchanger 
6 of which the temperature level can be regulated by means of a regulating 
device which is not shown here. The compartments are separated by 
partitions 9. In the last two compartments before the outlet, an exchanger 
10 has been placed and which may, for example, be a tubular exchanger 
which permits the plaster to be cooled while reheating the fluidization 
gas 4 or the primary furnace air by means of pipe 11. The transfer of the 
material during dehydration, from one compartment to the next, occurs by 
openings 12, the size and position of which can be adjusted and selected 
so that the material does not return to the compartment it has just left, 
and that the heating element is always immersed. The plaster is discharged 
by means of an overflow 13. A dust elimination device 15 is located at the 
outlet 14 of the steam-charged fluidization gases. A simple dust filter 
can be used for this purpose. Only extremely fine particles are entrained 
with the gases and are recycled through 16 in the reactor. Measurement and 
control apparatuses have also been installed but these have, for the sake 
of clarity not been illustrated. The apparatus is preferably constructed 
to have a very slight horizontal incline, the raised end being that for 
the intake of material. 
The pre-washed, purified (if necessary), and, usually dried, gypsum is 
introduced in the upper part of the reactor at the same time as the 
fluidization gas, which is generally air, is introduced in the power part. 
The size and position of the openings 12 connecting the compartments have 
been adjusted so as to allow the reacting mass to circulate 
unidirectionally. The rate of the fluidization gases is regulated so as to 
achieve a movement of material whereby each compartment behaves like that 
of a homogeneous reactor. 
The dehydration of the gypsum proceeds progressively as the material 
advances from one compartment to the next. For example, in an experiment 
carried out in a reactor comprising four calcination compartments, the 
change in the water content C of the product exiting from each compartment 
was recorded (the product originates from the manufacture of phosphoric 
acid and corresponds to that of Example 5 below). In addition, the 
temperature T of the product in each compartment was recorded. 
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Compartments 1 2 3 4 
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T, in .degree.C. 
125 150 168 175 
C, in % 15.4 11.4 8.7 5.9 
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Fluidization with a reduced rate is obtained, limiting dispersions to very 
fine particles, generally in an amount of the order of 5 to 15%. These 
particles are easily recovered in a purification device of a customary 
type, and are returned to the reactor. The presence of these fine 
particles helps in imparting qualities of reactivity to the finished 
product. The fluidization employed makes it possible to maintain in the 
reaction medium a relatively high mean water vapor pressure, generally of 
between 130 and 550 mm. Hg., a water vapor pressure of this order makes it 
possible to carry out the reaction at a high temperature level, which is 
also a factor favoring the reactivity of the plaster. Uniform fluidization 
is obtained which results in a high heat transfer coefficient between the 
heating element and the reacting mass material. In fact, heat transfer 
coefficients of the order of 250 to 400 kilocalories/hour/m.sup.2 /degree 
Centigrade are observed, this value varying with the rate of fluidization 
and the degree of fineness of the starting material. This is an excellent 
heat exchange and makes it possible to reduce the size of the 
fluidized-bed burner apparatus vis-a-vis the known devices, due to the 
selection of high temperatures on the walls of the hearing element. The 
system thus has the advantage of being compact. The system also permits 
maximum heat recovery. The volume of fluidized air utilized in the process 
is low. The heat input required to be generated by the boiler or heat 
exchanger to the system is not a great burden since there is but a small 
difference in temperature between the entry and exit of the fluid in each 
heating element. 
Furthermore, as a result of the rather high temperature of the heated fluid 
at the inlet of the exchanger of a particular compartment, an average high 
temperature difference is achieved between the heat transfer fluid and the 
reacting mass, which is both favorable to the effect of thermal impart on 
the grain and also eliminates the need for more than one relatively small 
exchange surface. In addition, the production capacity of the installation 
can be made to vary by varying the exchange surface by activating or 
deactivating one or more exchangers, the exchangers in service working at 
a high temperature level and fluidization conditions being maintained 
within similar limits, allowing for constant plaster quality. 
The heat balance of the process is improved still further by the 
possibility of reheating the fluidization air and/or the primary furnace 
air by cooling the plaster. The fumes from the boiler can also be utilized 
in another part of the installation. The use of low fluidization rate also 
makes it unnecessary to employ a gas blower of large capacity. 
The process and apparatus of the invention permit advantageous treatment of 
gypsums of any origin, both natural and synthetic, provided they are fine 
and dry. Natural gypsum which is crushed and then ground after extraction 
is generally a powdery product having particles of varying dimensions with 
fine grains of 10 to 20.mu., but also with particles of 200 to 500.mu.. 
Conventional heat treatment of natural gypsum encounters difficulties in 
relation to uniformity of heating and mechanical segregation. 
Synthetic gypsums such as those originating from the production of 
phosphoric acid are commonly called phosphogypsums. The phosphogypsums 
appear in aqueous suspension, and after drying, in powdery form of rather 
even grain size, most often ranging between 10 and 100.mu., the size of 80 
percent of the particles being between 25 and 75.mu.. The calcination of 
such a product creates a cleansing and dust elimination problem. 
It is obvious that regulating the temperatures of the various heating 
elements to different values and choosing higher fluidization speeds would 
make it possible to obtain, when required, products which have been 
dehydrated beyond the hemihydrate stage.

EXAMPLE 1 
A gypsum by-product from the manufacture of phosphoric acid is treated. 
After the purification, neutralization, filtration and drying operations, 
the pulverulent gypsum has the following characteristics: 
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H.sub.2 O 19.6% 
Gypsum (CaSO.sub.4 . 2H.sub.2 O) 
96% 
Particle size, cumulative 
125.mu. 
&lt;1% 
retention 
100.mu. 
2-4% 
80.mu. .about. 10% 
40.mu. .about. 60% 
25.mu. .about. 80% 
Apparent density 0.85. 
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850 kg/hour of this gypsum are introduced into an apparatus such as that 
shown schematically in FIG. 1, which comprises four calcination 
compartments. Measurements are taken of the temperatures of the gypsum, 
the rate of the fluidization gas, the temperature of the heat transfer 
fluid at the inlet and outlet of the calcination compartments in 
operation, the temperature of the product obtained at the outlet of the 
calcination compartments and at the outlet of the cooling compartment, the 
temperature of the fluidization air and the mean weight ratio of water/air 
(water resulting from dehydration relative to fluidization air); the 
proportion of circulating fines is 8%. 
These data are all given in Table 1, below, where the temperatures are 
expressed in degrees Centigrade and where .DELTA.t represents the mean 
temperature difference between the heated fluid and the plaster in the 
final calcination compartment. 
At the outlet of the apparatus, 710 kg/hour of plaster are collected having 
the following characteristics after grinding: 
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H.sub.2 O 5.6% 
pH of a 20% strength suspension 
6.3% 
Gypsum (non-calcined) 1.2% 
Hemihydrate 82-85% 
Anhydrite III 14-17% 
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Its mechanical properties are measured in accordance with Standard 
Specification NFB 12,401 and the results are summarized in Table II. 
This plaster is excellent for use in prefabrication. 
EXAMPLE 2 
A natural gypsum which has been crushed and grund is treated and has the 
following characteristics: 
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Total H.sub.2 O 23.1% 
CaSO.sub.4 2H.sub.2 O 96% 
Particle size, cumulative retention 
400.mu. 0% 
250.mu. 7% 
150.mu. 14% 
100.mu. 26% 
80.mu. 34% 
40.mu. 62% 
25.mu. 85% 
Apparent density 0.91 
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800 kg/hour of this gypsum are introduced into the same installation as in 
Example 1 and the same values as those shown in Table I are measured. At 
the outlet of the apparatus, 670 kg/hour of a product containing the 
following are collected: 
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Non-calcined gypsum less than 2% 
Hemihydrate 83-86% 
Anhydrite III 12-15% 
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The mechanical properties of the product are shown in Table II. 
This plaster is excellent for use in prefabrication. 
EXAMPLE 3 
(Comparative) 
Calcination of the same starting material as in Example 1 is carried out, 
but in a rotary furnace with indirect heating, this furnace being provided 
with a device which permits recycling of the fines into the bed of 
reacting material during the conversion process analogous to that of the 
preceding examples. 
A conversion yield to plaster of more than 98% is obtained. The mechanical 
properties of this plaster, measured as in the preceding examples, are 
given in Table II. It is seen that a conventional method of calcination 
does not make it possible to obtain the excellent mechanical properties of 
the phosphogypsums of Example 1. 
EXAMPLE 4 
(Comparative) 
A comparison of mechanical properties of a plaster which is obtained from a 
natural gypsum similar to that of Example 2, but which has undergone a 
conventional heat treatment is shown in Table II. It is seen that the 
product does not have as good reactivity characteristics as those of the 
product of Example 2 and that the product is less suited for 
prefabrication. 
TABLE I 
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Examples 1 2 
__________________________________________________________________________ 
Treated product CaSO.sub.4 . 2HO, 96% 
CaSO.sub.4 . 2H.sub.2 
O, 96% 
Origin from phosphoric acid manufacture 
natural 
Apparent density 0.85 0.91 
Number of compartments 4 4 
Fluidization gas air air 
Rate in cm/sec. 5.3 6.3 
Temperature of the gypsum at the inlet (in .degree.C.) 
60 18 
Inlet temperature of the fluid entering the compartments in 
200-285on 265-270 
Outlet temperature of the fluid 250-255 245-250 
Temperature of the plaster: 
on leaving the calcination 180-182 172-175 
.increment.t 86 84 
on leaving the cooling 112-115 110 
Temperature of the fluidization air: 
at the inlet 109 104 
at the outlet 135 130 
Weight ratio of water/air, in kg/kg 
0.85 0.53 
Fines recycled, % 8 10 
Yield of conversion to hemihydrate 
82-85 83-86 
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TABLE II 
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NFB 12,401 Ex. 1 Ex. 2 Ex. 3 Ex. 4 
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Start of setting (in minutes and 
2.20 4.10 3.40 6 
seconds) 
End of setting (in minutes and 
10.30 17.30 13 23 
seconds) 
Flexural strength (in bars) 
42 32 27 32 
Compressive strength (in bars) 
127 87 90 80 
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It can be seen that the plaster obtained from the phosphogypsum has 
mechanical properties superior to those of the plaster obtained from 
natural gypsum and treated according to the process of the invention. 
The setting time of the plaster obtained from the phosphogypsum is shorter 
than that of the natural gypsum and the onset of setting occurs sooner. 
While these rapid setting characteristics can be attributed to the greater 
proportion of fine particles in the phosphogypsum, and to the recycling of 
these fines, the accelerating effect of which is known, these 
characteristics are also due to the heat shock and to the possibility of 
regulating the water vapor pressure, both of which are present in the 
process of the invention. In Comparative Example 3 the process of 
calcination of the invention renders the phosphogypsum fines more reactive 
than do the conventional calcination processes. 
The comparison of Examples 2 and 4 shows that the process of the invention 
makes it possible to obtain, from natural gypsum, a product which is more 
reactive than the product obtained by the known calcination processes. 
The process of the invention applies, with all its advantages, to the 
thermal treatment of phosphogypsums supplying a product which is 
particularly suitable for prefabrication and with the additional advantage 
that grinding is not required. 
EXAMPLE 5 
To show progressive dehydration: 
The same starting material as that described in Example 1 is calcined, 
under the same conditions except for the following: 
Temperature of the gypsum at the inlet . . . 90.degree. C. 
Inlet temperature of the fluid entering the compartments in operation . . . 
270.degree.-275.degree. C. 
Outlet temperature of the fluid . . . 235.degree.-240.degree. C. 
The temperature of the plaster on leaving the calcination is 
172.degree.-175.degree. C. 
The temperature of the plaster on leaving the cooling is 
108.degree.-110.degree. C. 
In each compartment, the average .DELTA.t is: 
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Compartments 1 2 3 4 
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.increment.t 130 105 87 80 
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Pursuant to the requirements of the patent statutes, the principle of this 
invention has been explained and exemplified in a manner so that it can be 
readily practiced by those skilled in the art, such exemplification 
including what is considered to be the best embodiment of the invention. 
However, it should be clearly understood, that within the scope of the 
appended claims, the invention may be practiced by those skilled in the 
art and having the benefit of this disclosure, otherwise than as 
specifically described and exemplified herein.