Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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Process for the preparation of crystalline sodium sili-
cates having a sheet structure
The present invention relates to a process for the pre-
paration of crystalline sodium silicates having a sheet
structure and an SiO20/NazO molar ratio of 1.9 : 1 to 3.5
: 1 from waterglass solutions having a solids content of 20
- to 65% by weight.
In the process for the preparation of crystalline sodium
silicates according to U.S. Patent 4,585,642, a small
amount of crystalline sodium silicate is added to liquid or
solid sodium ~isilicate having a water content of 5 to 95%
by weight before water is removed from the reaction mixture
and the latter is kept at a temperature of 450C to just
below the melting point until the total amount of sodium
silicate has crystallized.
Furthermore, U.S. Patent 4,664,839 discloses that, among
the vario~s crystal modifications of crystalline sheet
silicates of the formula Na25i20s, the S-form has the
highest cation exchange power and is therefore particu-
larly suitable for softening water.
A disadvantage of the known process is that a solid bulky
silicate foam is formed during removal of water from the
amorphous sodium silicate. Moreover, during heating of
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the dehydrated sodlum sillcate, a temperature range of 580 to
630 C is employed, in which, owing to an exotherrnic reaction,
the sodium slllcate melts for a short tlme and forms extremely
hard, bulky aggregates. In both process steps, there ls
therefore the danger that contllluously operatlng apparatuses
wlll becorne blocked. Flnally, milllng of the partlcular
product, whlch ls requlred ln both process steps, entails
considerable expense.
It is therefore the obiect of the present lnventl.on
to provlde a process which permits trouble-free contlnuous
preparatlon of crysta]line sodium slllcate havlng a sheet
structure ln the ~-modlflcatlon from waterglass solutlons wlth
llttle mechanlcal comrninutlon.
Accordlngly, the present lnventlon provldes a process
for the preparatlon of crystalllne sodlum slllcate havlng a
sheet structure and an S102/Na20 rnolar ratlo ranglng from (1.9
to 3.5) sl frorn waterglass solution havlng a sodlum slllcate
content of 20 to 65% by weight comprlslng
a) spray-drying the waterglass solutlon ln a spray-
drylng zone to form an amorphous sodlum sllicate havlng a
maximum ignition loss of 20% by welght, exhaust gas leavlng
sald spray-drylng zone havlng a temperature of at least 140C.;
b) heating in an anne~ling zone the spray-dried
amorphous sodium silicate of step (a) ln a rotary tubular klln
incllned 1 to 5 from the horizontal at temperatures ranging
from 500 C. to 800 C. for 1 to 60 rnlnutes in order to effect
crystalllzatlon of said amorphous sodlum slllcate; and
c) recovering sald crystalllne sodlum~sillcate from sald
rotary tubular klln;
the lmprovement consistlng essentlally of addlng 10
welght-% to 50 weight-%, based on the weight of sald amorphous
sodiutn sillcate, of crystal]ir,e s'odium silicate obtalned by
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mechanlcal commlnution to the anneallng zone in step b) so as
to prevent adherence of any materlal to the walls of sald
- rotary tubular klln and thereby enable contlnuous recovery of
pulverulent product ln step (C).
It ls also posslble
a) for up to 50% by weight of crystalllne sodlum
sillcate dlscharged from the lgnitlon zone to be recycled to
the ignltion zone after rnechanical comminution;
b) for the mechanically comrnlnuted, crystalline sodium
slllcate to have partlcle sizes of 10 to 1,000 ~m;
c) for spray-drying of the waterglass solutions and
heatlng of the sodlum silicate to be carried out together in a
directly fired rotary tubular kiln;
d) for the waterglass solutlons to ~e sprayed in at the
non-fired end of the rotary tubular klln, whlle the heated
sodlum slllcate emerges at the flred side of the rotary tubular
klln;
e) for the rotary tubular klln to be inclined 1 to 2 to
the horlzontal;
f) for the spray-dried sodlurn sillcate to have a maxlmum
lgnltlon loss of 5% by welght;
g) for the amount of crystalllne sodlum slllcate
recycled to the ignltion zone to be the greater the hlgher the
lgnitlon loss of the spray-dried sodium slllcate.
The crystalllne sodlum slllcates obtalned uslng the
process accordlng to the lnventlon have pH values of 10.0 to
10.5 and a calclum-blndlng power of more than ~60 meq Ca/100 g
(at 20C) or more than 600 meq ca/100 g ~at ~0C) whlle their
magneslum-blnding power in the same pH range ls more than 580
meq Mg/lOOg (at 20C) or rnore than 1,000 rneq Mg/lOOg (at 60C).
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....
In the process according to the invention, the quality of
the resulting amorphous sodium disilicate powder can be
influenced ;n a wide range in the course of the spray-
S drying by changing the concentration of the waterglasssolution and by controlling the spraying temperature. Thus
for example, the amorphous sodium silicate powders to be
treated according to the invention in the ignition zone
and having a water content of 1 to 20% by weight can be
prepared in a hot-air spray tower from waterglass solutions
having a modulus (SiO2 : Na20 ratio) of 2.
Advantageously, the process according to the invention
can be carried out in a single apparatus which permits
the steps comprising spraying of the waterglass solution,
heating in an agitated bed and recycling of the crystal-
line sodium silicate into the ignition zone. This can
be carried out in a fluidized bed reactor or a rotary
tubular kiln operated with hot gas, into which waterglass
solution is sprayed and into which crystalline sodium
silicate is simultaneously metered in. A rotary tubular
kiln fired directly with oiL or gas is preferred, in which
case the feed and discharge can be arranged at different
positions, and, depending on the inclination of the
furnace with respect to the horizontal, discharge is
effected after a shorter or longer heating time.
In the examples which follow and in which the invention
is described in detail, the calcium- and mlagnesium-binding
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power of the resulting crystal(ine sodium silicates hav-
ing a sheet structure are determined as follows:
S Solutions of CaCl2 (corresponding to 300 mg of CaO) or
MgCl2 (corresponding to 216 mg of MgO) are added to 1 l
of distilled water, with the result that a water having
30 German hardness was obtained.
1 9 of the crystalline sodium silicate obtained in Exam-
ples 2 to 7 and O to 6 ml of 1-molar glycine solution
~obtained from 75.1 9 of glycine and 58.4 9 of NaCl
which were dissolved in water and made up to 1 l) were
added to 1 l of this water, which had been heated to
either 20 or 60'oC, and the pH value was then adjusted to
10.4. The suspension was stirred for 30 minutes, during
which the pH remained stable. Finally, the solution was
filtered and the calcium and magnesium remaining in solu-
tion were determined complexometrically in the filtrate.
The calcium- and magnesium-b;nding power were determined
by calculating the diference with respect to the original
contents.
The results for Examples 2 to 7 are summarized in the
attached table.
Example 1 (Comparative Example)
, . ..
Amorphous sodium disilicate which had an i~nition loss
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of 19% was produced from a waterglass solution having a
solids content of 45X and a modulus of 2 in a hot-air
spray tower (waste gas temperature: 145C). The amorphous
sod;um disilicate was metered into the end wall of a
rotary tubular kiln heated electrically from the outside
(length: 3 m; diameter: 22 cm; inclination: 1.6)
via a metering screw at a rate of 2 kg~h, the residence
time in the furnace being about 45 minutes and the tem-
perature at its hottest point be;ng 720C.
After the material had initially expanded considerablyin the rotary tubular kiln, it began to adhere to the
walls on reaching the zone at about 550C, large leaves
being formed, and rolling up to g;ve lumps of about 10 cm
diameter. The rotary tubular kiln was blocked by the
lumps to such an extent that the material flow in the fur-
nace could be maintained only by constant poking. After
an operating time of 2 hours, the cross-section of the
rotary tubular kiln was virtually completely blocked, so
that the experiment had to be discontinued.
Example 2 (Comparative Example)
The amorphous sodium disilicate was prepared as in Example
1. The amorphous sodium disilicate was fed via a meter-
ing screw into a directly fired rotary tubular kiln
(length: 5 m; diameter: 78 cm; inclination: 1.2)
at its end opposite the flame, while the crystalline
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product was discharged at the flame end. 25 kg/h of amor-
phous sodium disilicate were metered; the temperature at
the hottest point of the rotary tubular kiln was 740C.
s
Material adhered to the wall of the rotary tubular kiln
and had to be forced off mechanically. AggLomerates formed
having a diameter up to about 20 cm.
Example 3 (Comparative Example)
The procedure was similar to Example 2; however, 60 kg/h
of amorphous sodium disilicate and at the same t;me 5 kg/h
of a recycled material obta;ned by comm;nut;ng the product
obta;ned ;n Example 2 to less than 250 ~m were metered.
Material adhered only weakly to the wall of the rotary
tubular k;ln and could be removed by occas;onal tapping.
The largest agglomerates occuring had a diameter of about
8 cm.
Example 4 (according to the invention)
Example 3 was repeated with the modificat;on that 15 kg/h
of recyc(ed mater;al were metered.
No mater;al adhered to the wall of the rotary tubular kiln
the crytall;ne sodium silicate discharged was substan-
t;ally pulverulent. `
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Example 5 (according to the invention)
A waterglass solution hav;ng a solids content of 55% and
a modulus of 2 was spray-dried in a hot-air spray tower,
the waste gas temperature being 230C and an amorphous
sodium disilicate having an ignition loss of 4.7% being
obtained.
The amorphous sodium disilicate was metered at a rate of
40 kg/h, together with 4 kg/h of recycled material, into
a gas-fired rotary tubular kiln (inclination: 1.2).
No caking occurred in the rotary tubular kiln; the dis-
charged crystalline sodium silicate was substantially
pulverulent.
Example 6 (according to the invention)
The waterglass solution according to Example 5 was sprayed
through the flame of a directly fired spray tower. An amor-
phous sodium disilicate having an ignition loss of 1.4X
was obtained at a waste gas temperature of 450C. The
amorphous sodium disilicate was heated together with the
recycled material, as stated in Example 5. In this case
too, no caking occurred in the rotary tubular kiln and a
substantially pulverulent sodium silicate resulted.
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Example 7 (according to the invention)
The rotary tubular kiln described in Example 2 was addi-
tionally equipped, on its product-inlet s;de, with a spray
system through wh;ch 50 l/h of a 50% strength waterglass
solution were sprayed. At the same t;me, 5 kg/h of re-
cycled material were introduced via a sol;ds meter;ng
system, cocurrently w;th the sprayed waterglass solut;on~.
The waste gas temperature was 220C and the temperature
' .. 't,: ~ 10 at the hottest po;nt of the rotary tubular kiln was 750C.
The pr;mary spray product had an ignition loss of 4.8Z.
No material adhered to the wall of the rotary tubular kiln.
The largest agglomerates ;n the d;scharged crystall;ne
sod;um had a d;ameter of about 3 cm.
Table
Calcium- and magnesium-binding pouer of crystaLline sodium
silicates having a sheet structure at pH 10.4
According to Example Calcium-binding power CmgCa~g~ Magnesium-binding power [mgMg/g~
at 20~ at 60 C at 20C at 60C
2 68 114 66 120
3 72 120 70 t24
4 74 123 72 128
78 126 74 130
6 76 - 124 74 130
7 75 124 73 128
