Note: Descriptions are shown in the official language in which they were submitted.
CA 02209~89 1997-07-04
Process for preparing virtually phase-pure calcium
sulphate ~-hemihydrate
The invention relates to a process for preparing
virtually phase-pure calcium sulphate ~-hemihydrate.
Calcium sulphate ~-hemihydrate is a calcium
sulphate phase which is formed by calcination of calcium
sulphate dihydrate under industrial conditions in a
temperature range from 125 to 180~C.
The raw material used for preparing calcium
sulphate ~-hemihydrate has in the past been pre~om;n~ntly
granular natural gypsum. At present, finely divided raw
materials such as flue gas desulphurization gypsum or
chemical gypsum are used to an increasing extent, and
their proportion will increase significantly in the
future.
To carry out the calcination, the following
apparatus, for example, are in use:
Rotary tube furnaces, heated -directly and
indirectly,
Milling-firing calciners,
Carrier gas calciners,
Heating apparatus (batchwise/continuous).
The calcination products are generally multiphase
mixtures which, dep~n~;ng on the respective process,
contain not only calcium sulphate ~-hemihydrate but also
varying amounts of calcium sulphate dihydrate and of
highly reactive, hygroscopic calcium sulphate anhydrite-
III which reacts in air to form calcium sulphate
~-hemihydrate.
However, for the production of modern building
materials, for example plaster, it is necessary to
produce, as base calcium sulphates, calcination products
which have a high phase purity (in particular free of
calcium sulphate dihydrate), uniformity and stability.
The freedom from calcium sulphate dihydrate of
base calcium sulphates is at present ensured by high
temperatures and long residence times during the
calcination process, with the acceptance of the formation
CA 02209~89 1997-07-04
of calcium sulphate anhydrite-III which is only
unsatisfactorily eliminated in downstream process steps.
In addition, DE 28 15 792 Al discloses subjecting
finely divided calcium sulphate dihydrate originating
from phosphoric acid production, which contains foreign
bodies of solid solution, after surface washing to a two-
stage heat treatment in a calcination apparatus. In this
process, the calcium sulphate dihydrate is heated in a
first stage at a temperature of from 110 to 150~C for
from 30 to 90 minutes and then conditioned in a second
stage at 130 to 180~C for from 10 minutes to a nl~mher of
hours, where the second and possibly also the first stage
are carried out at a partial pressure of water vapour of
~ 133 mbar. The conditioning is carried out in the
presence of lime in order to eliminate, by means of
d~m; ~; n~, the influences of ions of foreign materials
which originate from the phosphoric acid production,
with, in particular, water-cont~;n;ng dibasic calcium
phosphate being converted into its anhydrous form which
has only a very low solubility. The lime becomes
effective only in the presence of a certain degree of
humidity, which is why the process is carried out at the
partial pressure of water vapour mentioned. This allows
the production of a calcium sulphate which is equivalent
to natural calcium sulphate and is thus a phase _ixture.
It is an object of the invention to develop a
process which makes it possible to prepare very phase-
pure calcium sulphate ~-hemihydrate.
This object is achieved by means of the features
of Claim 1.
In this process, finely divided calcium sulphate
dihydrate is heated in a first stage to a temperature of
from about 110 to 140~C, preferably from about 120 to
130~C, held at this temperature for a defined residence
time and then, in a second stage, held at a temperature
of from 110 to 140~C, preferably from 120 to 130~C, under
a defined water vapour atmosphere until virtually phase-
pure calcium sulphate ~-hemihydrate is present. The
moisture content of the calcium sulphate dihydrate used
CA 02209~89 1997-07-04
as raw material is here ~ 0.2% by mass.
It has been found that in the calcination of
finely divided, dry calcium sulphate dihydrate, when the
water of crystallization content is plotted versus the
reaction time there occurs, under certain conditions, a
plateau at a water of crystallization content of from
about 7 to 6.5% by mass based on anhydrite; cf.
accompanying graph. The water of crystallization content
is dependent on the purity of the calcium sulphate
dihydrate which in the case presented is ~ 97% by mass.
The curves in the graph show the water of crystallization
content as well as the material temperature and the
partial pressure of water vapour during the reaction. In
the graph, the material temperature in ~C and the partial
pressure of water vapour in 10 mbar are shown on the
left-hand ordinate, the water of crystallization content
in % by mass (based on anhydrite) is shown on the right-
hand ordinate and the reaction time in hours:minutes is
shown on the abscissa.
When the starting material is heated in the first
stage to from about 110 to 140~C, preferably from 120 to
130~C, and is held at this temperature for a defined
residence time of advantageously from about 20 to
180 minutes at a defined partial pressure of water
vapour, a phase mixture of calcium sulphate ~-hemihydrate
and calcium sulphate dihydrate having a water of crystal-
lization content of from about 6 to 9% by mass,
preferably from about 7 to 8% by mass, is obtained
(dep~n~;ng on the calcium sulphate dihydrate purity, here
~ 97%). In order to obtain virtually phase-pure calcium
sulphate ~-hemihydrate, the calcium sulphate dihydrate
- still present has to be converted into calcium sulphate
~-hemihydrate. For this purpose, the phase mixture is, in
the second stage, held at low material temperatures and
a high, defined water vapour content until almost phase-
pure calcium sulphate ~-hemihydrate is obtained.
Subsequent cooling of the calcined material to
about 50~C or less, if desired under an atmosphere of
water vapour, i8 advantageous.
CA 02209~89 1997-07-04
However, the calcined material can also, if
desired, be cooled in a third step to a temperature in
the range from about 40 to 70~C, in particular from 50 to
60~C, and at the same time or partly at the same time or
after cooling held at this temperature for from about
10 minutes to 2 hours, in particular from 30 minutes to
60 minutes, in a defined water vapour atmosphere. The
partial pressure of water vapour is to be selected such
that it is below the m~Yim-lm value at the hold
temperature, for example at from 30 to 90% of the m~Y;mllm
value at the intended hold temperature, in order to avoid
going below the dew point.
Cooling can be carried out by means of screws,
drums, rotary tube apparatus or fluidized-bed apparatus
and the like.
If milling of the calcined product is to follow,
it is advantageous if the temperature of the product is
here held at less than about 50~C.
This process can be implemented on an industrial
scale 80 that in this way large amounts of virtually
phase-pure calcium sulphate ~-hemihydrate free of calcium
sulphate dihydrate can be produced continuously such that
~ 98% by mass of the calcium sulphate dihydrate present
in the raw material have been converted into calcium
sulphate ~-hemihydrate.
Suitable raw materials are, in particular, finely
divided calcium sulphate dihydrate having a ~-Y;ml-
~particle size of 100 ~m, for instance flue gas
desulphurization gypsum or chemical gypsum. These gypsums
are obtained in moist form and have to be dried to a
moisture content of ~ 0.2% by mass before calcination.
The process can be carried out in rotary tube
furnaces. In these, the heating medium can be conveyed in
cocurrent to, transverse to or in countercurrent to the
material stream. An appropriate residence time of the
material at the required temperature in the intended
intervals can be set by means of an appropriate
structural configuration of the rotary tube furnaces, for
example weirs, shutters, helical screws or the like.
CA 02209~89 1997-07-04
-- 5
In a rotary tube furnace, the first stage of the
process can first be carried out. By means of appropriate
provision of different temperature zones in such a rotary
tube furnace, the second stage of the process can also
5 follow in this. If desired, a lock can be provided which
serves to divide the rotary tube furnace into two regions
for the first and second treatment stages. However, it is
preferable to carry out the second stage in a different
apparatus; it can be carried out batchwise in appropriate
heating apparatus or continuously in a further rotary
tube furnace in which it is only necessary to hold the
appropriate temperature and maintain a defined water
vapour atmosphere.
In place of rotary tube furnaces, it is also
possible to use fluidized-bed or moving-bed apparatus as
continuous throughput apparatus for the calcination.
The first stage is associated on the one hand
with high water vapour evolution and on the other hand
with considerable whirling up of dust so that the water-
containing vapours discharged from the first stage of theprocess are heavily laden with dust. The dust can, for
example, be separated out via a cyclone and added to the
material for the second stage.
In addition, water vapour from the first stage
can be used to set a defined water vapour atmosphere in
the second stage, preferably having a partial pressure of
water vapour in the range from about 200 to 900 mbar. In
the first stage too, a partial pressure of water vapour
in the range from about 200 to 900 mbar is preferably
3 0 maintained by appropriate discharge of vapours.
In order to obtain a defined temperature-
residence time profile of the material in the two stages
of the process, it is advantageous to convey the material
by means of a screw or the like.
The average residence time of the material in the
second stage of the process is advantageously from about
10 to 120 minutes, preferably from about 30 to
60 minutes.
CA 02209~89 1997-07-04
E x a m p 1 e
As raw material calciumsulfate dihydrate in the form of fine
particles originating from flue gas desulfurization was dried to a
moisture content of < 0.1 weight~~ and had a purity of 98%.
In a first calcinating step (precalcination) within a rotary
kiln the raw material was calcinated during 40 minutes at a temperature
of 130~C and 0.2 bar of water vapor partial pressure.
This was followed by a second calcinating stept within the ro-
tary kiln, according to which the precalcinated material from the first
calcinating step was calcinated during 100 minutes at a temperature of
130~C and 0.7 bar of water vapor partial pressure.
The resulting product had the following phase composition
(related to the 98% dihydrate content of the raw material):
calciumsulfate ~-hemihydrate: 100 weight%
calciumsulfate anhydrite-III: not detectable
calciumsulfate dihydrate: not detectable.
