Note: Descriptions are shown in the official language in which they were submitted.
~1 3~8921
The present invention relates to a method of processing
calcium sulfate alpha hemihydrate.
Various processes for producing calcium sulfate alpha
hemihydrate are known, these being the so-called wet processes,
in which crude gypsum is fed continuously into an autoclave in
the form of a suspension and converted to calcium sulfate alpha
hemihydrate, and autoclaving processes in which the crude gypsum
is placed in an autoclave as fragments or as a shaped body. As
an example, in the production of calcium sulfate alpha
hemihydrate as crude gypsum that originates from the desulfuring
plants of power stations and is compressed into shaped bodies, it
is possible to obtain large, regular crystals (primary grain)
after the autoclaving process. The shaped bodies, which are of
the primary grain, are broken up in a breaker and the primary
grain is thereby reduced to a coarse-grain product with a
specific surface area of, for example, 800 cm2/g.
Such a coarse grain calcium sulfate alpha hemihydrate is a
valuable product per se because of its low water requirement
when it is used to produce mortar or the
like. However, the coarse grain does not lead to the rapid
development of strength since the conversion per unit time of the
coarse grain with water is relatively small or incomplete. The
usual accelerators can be used in order to speed up this
conversion. However, these entail the disadvantage that they
result in quick set up. It is
1 338921
true that this situation can be improved by adding extra water;
however, additional mixing water is not desirable. In addition,
in the normal course of events, one requires some lead time in
order to mix the mortar and deliver it to the site where it is
needed.
It is the task of the present invention to create a process
of the type described in the introduction hereto, that permits
the production of calcium sulfate alpha hemihydrate that has a
relatively slow onset time for setting up, and which despite this
leads to the rapid development of strength and to a high level of
strength.
This task has been solved in that the calcium sulfate alpha
hemihydrate that is present in the form of primary grain is
brought to a degree of fineness that corresponds to the
development of strength that is required by the intended purpose
the finest grain being removed by screening, and in that at least
one fruit acid and/or
the salt(s) thereof is added as a retarder and liquefier.
In particular, specific surface areas in the range from
approximately 1200 to 4000 cm2/g, especially 1500 to 3500 cm2/g
are arrived at by grinding the coarse grain starting product. A
specific grain size range can be obtained by
screening, so that both the finest grain sizes and the coarsest
grains sizes are eliminated. The very finest grain size is very
reactive and has a very great water requirement, so that the
desired properties can be restricted by it. Although coarser
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grain sizes do not require large quantities of water, they are not
sufficiently reactive.
The fruit acids, such as malic acid, acetic acid, tartaric
acid, and in particular citric acid, or the water-soluble salts
thereof, which can be used either alone or in combination, delay
the onset of the setting up process and also serve as liquifiers,
the effects of which occur immediately if the calcium sulfate
alpha hemihydrate comes into contact with the mixing water. Quite
apart from this, the compression strength is greatly increased
10 thereby.
This latter fact is surprising in that up to now, citric acid
has been considered to be a retarding additive that reduces
strength if used in small quantities of less than 0.1 M.-~, (wt %)
whereas in larger quantities it even prevents consolidation (see
Ullman's EncYklopadie der technischen Chemie, 4th ed., Vol. 12,
p. 307; R.A. Kuntze, "The Chemistry and Technology of Gypsum,"
ASTM Special Technical Publication 861, 1984) so that citric acid,
like the other related fruit acids, has up to now been considered
undesirable.
The fruit acid is used in particular in a quantity from 0.005
to 0.05 M.-%, preferably in a quantity of approximately 0.02 M.-%,
as citric acid monohydrate.
In the drawings Figure 1 is a graph illustrating the setting
of various alpha hemihydrate paste specimens against time and
~igure 2 is a graph illustrating the strength of various alpha
hemihydrate paste specimens against time.
Reference will now be made to the drawings in further
illustration of embodiments of the invention.
Figure 1 is a diageam that shows the stiffening of calcium
30 sulfate alpha hemihydrate paste as a function of the grinding
fineness, the needle distance in mm of a Vicat apparatus being
shown on the ordinate, based on DIN 1168, and the abscissa
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s
showing the time in minutes. It can be seen that as the specific
surface area sp increases, the onset of stiffening is displaced
towards shorter times. However, the onset of stiffening of a
paste of calcium sulfate alpha hemihydrate with a specific
S surface area of 3000 cm2/g is shifted from approximately 7
minutes to approximately 30 minutes by the addition of 0.02 M.-%
citric acid monohydrate.
As can be seen from figure 2, the fineness of the grind
affects the development of strength. In figure 2 the ordinate
shows the compression strength in N/mm2 and the abscissa shows
the time in hours, the respective Curves A to E representing the
development of strength for pastes of calcium sulfate alpha hemi-
hydrate with the quoted specific surface areas from 800 to 4700 cm2/g in each
instance with the addition of 0.02 M.-% citric acid monohydrate.
As can be seen from this, the finest grain (specific surface area
4700 cm2/g, grain size 99% below 0.003mm) leads to a reduction of
compression strength in contrast to a binding agent grain size
with 3000 or 3500 cm2/g. The water requirement as expressed by
the water/gypsum ratio amounts in these cases to 0.28.
In addition to this, figure 2 shows for comparison the
strength curve for a paste with a specific surface area of 700 to
900 cm2/g (water/gypsum ratio 0.31) and 3500 cm2/g (water/gypsum
ratio 0.35), see respective curves F,G, although without the addition
of fruit acid. Here, the strengths that are obtained are
considerably below those that can be obtained with the addition
of fruit acid. It can also be seen that grinding the coarse
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grain material affects the strength curve in a disadvantageous
manner when there is no addition of fruit acid.
The development of strength can be controlled accordingly by
using calcium sulfate alpha hemihydrate with varying degree of
grinding fineness.
It is preferred that the fruit acid and/or the salts thereof
be mixed into the ground and screened calcium sulfate alpha
hemihydrate in fine grain form, although it is also possible to
dissolve these in the mix - water and arrive at the desired
strength development and level of strength thereby. In addition,
salts of the fruit acids can be introduced into the product
through the preceding production process, in that the fruit acid
or the salt thereof is introduced as an additive that affects
growth during the recrystallization of calcium sulfate dihydrate
to alpha hemihydrate. In particular the process is suitable
during the use of alpha hemihydrate that has been produced by
recrystallization from dihydrate in an autoclave during the use
of a growth affecting additive in the form of finely ground brown
coal and/or peat and/or finely ground wood and/or humic acid
and/or wood constituents that have an equal effect
preferably with a
particle size of less than 100 ~m. This can be yLOUlld brown coal
at a quantity o~ 0.1 to l.OM.-%, preferably 0.5 to 0.7M.-%.
Ground peat can be used at a quantity of 0.1 to 1.5 M.-%,
preferably 0.5 to l.OM.-%. Wood consitituents that have an
equal effect also includes ground wood that can be added at a
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rate of 0.3 to 2.0 M.-%, preferably 0.7 to 1.5 M.-~. Humic
acid can be added at a quantity of 0.1 to l.OM.-%, preferably
0.3 to 7M.-% as a wood constituent. Sulfite spent lye
functions at a quantity of 0.1 to 3.0 M.-%, preferably from 0.5
s to 2 M.-%, as an additive that affects growth. Constituents
with an equal effect, from sulfite spent lye or secondary
conversion products of native lignin, such as lignin sulfonic
acid, for example, can be used in a quantity of 0.1 to 1.5M.-%,
preferably 0.3 to 1.2M.-%. Finally, these additives can be
lignin sulfonates in a quantity from 0.1 to 1.2M.-%, preferably
from 0.3 to 0.8 M.-%. A further growth affecting measure is
that a portion of the calcium sulfate dihydrate that is to be
converted consists ofwaste gas desulfurisation gypsum from a
power station that is fired by brown coal, e.g., in a quantity of
at least 25 M.-%.
The calcium sulfate alpha hemihydrate processed in this
manner is particularly suitable for the production of
pneumatically delivered spray mortar, that is wetted with water
on delivery, for immediate or imminently supporting dam
construction material used in underground workings.
