Note: Descriptions are shown in the official language in which they were submitted.
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1- HOE 90/H 006 K
The present invention relates to a process for producing
amorphous sodium silicates having a water content of 0.3
to 6 percent by weight, preferably of 0.5 to 2 percent by
weight, and an SiO2/Na2O molar ratio of (1.9 to 2.8) : 1
from a waterglass solution contA;ning at least 20 percent
by weight of solids.
From US Patent 3,471,253, it is known to obtain a water-
glass solution by introducing 42 percent by weight sodium
hydroxide solution and sand (silica) in a weight ratio of
about 2 : 1 into a stirred autoclave and allowing the
mixture to remain therein for 3 hours at 210C and 16
bar. The hot sodium silicate solution taken out after
cooling of the autoclave content to 85C contains, after
excess sand and other impurities have been filtered off,
57.5% of solids and has an SiO2/Na2O ratio of 1.64 : 1.
Crystalline sodium silicates having a layer structure and
an SiOz/Na2O molar ratio of (1.9 to 3.5) : 1 are produced
by the process according to German Offenlegungsschrift
3,718,350 by treating waterglass solutions having a
solids content of 20 to 65 percent by weight in a spray-
drying zone to form a water-contAining amorphous sodium
silicate, the exit gas flowing out of the spray-drying
zone having a temperature of at least 140C. The water-
cont~;ning amorphous sodium silicate is heat-treated in
an ignition zone for 1 to 60 minutes at 500 to 800C in
the presence of at least 10 percent by weight of recycle
material, which was obtained by mechanical comm;nlltion of
crys~All;ne sodium silicate previously discharged from
the ignition zone.
A disadvantage in the lastmentioned process is that the
material obtained in spray-drying takes up a large volume
because of its low bulk density of 100 to 250 g/l and
generates a lot of dust. Moreover, the use of recycle
material during the heat treatment causes considerably
greater expense on equipment and, because of the higher
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throughput of material, requires a rotary tube of greater
~;mensions.
According to the invention, the said disadvantages in the
production of amorphous sodium silicates from a water-
glass solution contA;n;ng at least 20% by weight of
solids are overcome by
a) obt~in;ng the waterglass solution by reacting quartz
sand with sodium hydroxide solution in an SiO2/Na20
molar ratio of (2.0 to 2.8) : 1 at temperatures from
180 to 240C and pressures from 10 to 30 bar,
b) treating the waterglass solution in a spray-drying
zone with hot air at 200 to 300C for a residence
time of 10 to 25 seconds and at a temperature of the
exit gas leaving the spray-drying zone of 90 to
130C, to form a pulverulent amorphous sodium
silicate having a water content (det~rm;ned as the
loss on ignition at 700C) of 15 to 23% by weight
and a bulk density of more than 300 g/l,
c) introducing the pulverulent sodium silicate accord-
ing to b) into an obliquely arranged rotary kiln
fitted with devices for moving solids and treating
it therein with flue gas in counter-current at
temperatures from 250 up to 500C for 1 to 60
minutes, the rotary kiln being insulated in such a
way that its outside wall temperature is less than
60C, and
d) comminuting the amorphous sodium silicate emerging
from the rotary kiln by means of a mechAn;c~
crusher to grain sizes of 0.1 to 12 mm.
Furthermore, the process according to the invention can,
if desired, also be further developed by
aa) grinding the co~~;nllted sodium silicate by means of
a mill to grain sizes of 2 to 400 ~m;
bb) using a mechanical mill running at a circumferential
speed of 0.5 to 60 m/second;
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cc) using an air jet mill;
dd) using a ceramically lined ball mill;
ee) using a ceramically lined vibratory mill;
ff) extracting the exit gas from the rotary kiln in the
central region thereof and in the region of the end
where the pulverulent amorphous sodium silicate
having a water content of 15 to 23% by weight is
introduced, and purifying the exit gas by means of
a dry dust filter, the sodium silicate taken from
the dry dust filter being quasi-continuously admixed
to the pulverulent amorphous sodium silicate des-
tined to be introduced into the rotary kiln;
gg) feeding the ground sodium silicate to a roll compac-
tor, by means of which it is compressed at a roll-
pressing force of 20 to 40 kN/cm of roll width to
give compact pieces;
hh) processing the compact pieces, after pre-comminution
by forcing them through screens, to give granules
having a bulk density of 700 to 1000 g/l.
Sodium silicates can be used as water-softening agents.
In the process according to the invention, a sodium
silicate of high bulk density which can readily be
handled, is obtained owing to the low temperature and the
short residence time in the spraying of the waterglass
solution.
Due to the low heat transfer through the wall of the
rotary kiln because of its good insulation, the ten~ency
of the sodium silicate to stick is counteracted in the
process according to the invention.
In the process according to the invention, the use of a
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low-speed mechanical mill tfor example a disk mill,
beater mill, hammer mill or roll mill) is necessary in
order to avoid abrasion of iron from the grinding tools.
If a ceramically lined ball mill or a vibratory mill or
an air jet mill for very fine products, i.e. those having
diameters of 6 to 10 ~m is used in the process according
to the invention, likewise no contAm;n~tion of the sodium
silicate due to metal abrasion occurs.
In the process according to the invention, the dust
loading in the exit gas is considerably reduced by the
simultaneous extraction of dust-cont~;n;ng exit gas in
the central region of the rotary tube and in the region
of its charging end, ~ecause dust is released above all
during charging of the sodium silicate to the rotary kiln
and because the gas velocity is reduced in the region
where the amorphous, water-cont~;n;ng sodium silicate is
charged.
Using the process according to the invention, an
abrasion-resistant granulated product, which very quickly
disintegrates in water, is obtained by compacting.
The residual hardnesses indicated in Examples 2 and 3
were det~rm;ned by the following procedure:
2.5 g of sodium silicate were suspended in 1000 ml of tap
water of 18 German hardness (corresponds to a content of
85 mg of Ca and 15 mg of Mg per liter). The suspension
was stirred for 30 minutes at 60C by means of a magnet
stirrer at about 500 rpm. After rapid cooling in ice
water to 20C, the suspension was filtered through a
membrane filter (pore width: 0.45 ~m). The calcium and
magnesium contents in the clear filtrate were determined
by means of atomic absorption.
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Example 1 (according to the state of the art)
In a hot-air spray tower (exit gas temperature: 145C),
amorphous sodium disilicate having a 1088 on ignition of
19~ at 700C and a bulk density of 220 g/l was produced
rom a waterglass solution having a solids content of
45%.
60 kg/hour of amorphous sodium disilicate having a water
content (determined as the 1088 on ignition at 700C) of
18% by weight and 15 kg/h of a recycle material, which
had been obtained by comminution of a product, obtained
in a previous batch, to less than 250 ~m, were charged
via a metering screw to a directly fired rotary kiln
(length: 5 m; diameter: 78 cm; inclination: 1.2) at its
end opposite the flame, while the crystalline product was
discharged from the flame side. The temperature at the
hottest point in the rotary kiln was 740C.
No material sticking to the wall of the rotary kiln was
formed; the discharged sodium disilicate was largely
pulverulent.
~Ample 2 (according to the invention)
Sand (99 percent by weight of SiO2; grain size 90%
< 0.5 mm) and 50 percent by weight sodium hydroxide solu-
tion in an SiO2/Na2O molar ratio of 2.15 : 1 were filled
into a nickel-lined cylindrical autoclave fitted with a
stirrer device. With the autoclave being stirred, the
mixture was heated to 200C by injecting steam (16 bar)
and held for 60 minutes at this temperatùre. The content
of the autoclave was then let down through a flash vessel
into a tank and, after the addition of 0.3% by weight of
perlite as a filter aid, filtered at 90C through a disk
pressure filter to separate off the insoluble matter. As
the filtrate, a clear waterglass solution having an
SiO2/Na2O molar ratio of 2.04 : 1 was obtained. The solids
content was adjusted to 50% by dilution with water.
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The waterglass solution was sprayed in a hot-air spray
tower which was fitted with a disk atomizer and which was
heated via a gas-fired combustion chamber and connected
to a pneumatically cleaning hose filter for precipitating
the product, the combustion chamber having been adjusted
in such a way that the hot gas entering at the tower top
had a temperature of 260C. The rate of the waterglass
solution to be sprayed was adjusted such that the tem-
perature of the silicate/gas mixture leaving the spray
tower was 105C. The residence time was calculated to be
16 seconds from the volume of the spray tower and the gas
throughput through the spray tower. The amorphous sodium
disilicate precipitated on the hose filter had, at a low
dusting te~ency, a bulk density of 480 g/l, an iron
content of 0.01~ by weight, an SiO2/Na2O ratio of 2.04:1
and a water content (deterrined as the loss on ignition
at 700C) of 19.4%; its mean particle diameter was 52 ~m.
The rotary kiln described in Example 1 had been insulated
by several plies of mineral wool and a sheet metal jacket
in such a way that, at a temperature of 390C in the
interior of the rotary kiln, a ~-~imllm temperature of
38C occurred on its outer skin. 60 kg of the amorphous
sodium disilicate were introduced per hour into this
rotary kiln, no sticky material being formed. The amor-
phous sodium disilicate (Na2Si2O5) leaving the rotary kiln
and showing a water content of 0.7% by weight (determined
as the loss on ignition at 700C) was comminuted by means
of a mechanical crusher to a grain size of less than 6 mm
and, after interm~Ai~te cooling, ground on a disk mill
(diameter: 30 cm) at 400 min~1 to a mean particle diameter
of 95 ~m, the iron content of the ground product being
identical to that of the sodium disilicate introduced
into the rotary kiln.
The exit gas from the rotary kiln was extracted in the
region where the amorphous sodium disilicate having a
water content (det~rmin~A as the loss on ignition at
700C) of 19.4% by weight was introduced, and fed to a
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scrubbing tower. 3 kg of sodium disilicate per hour were
discharged with the exit gas.
The residual hardness of the sodium disilicate thus
produced was 2.5 mg/l of Ca and less than 1 mg/l of Mg.
Example 3 (according to the invention)
Example 2 was repeated with the modification that the
temperature was 300C in the interior of the rotary kiln
and 35C on its outer skin. The amorphous sodium disili-
cate leaving the rotary kiln here had a water content
(determined as the loss on ignition at 700C) of 5% by
weight. The residual hardness of the sodium disilicate
produced in this way was 3.5 mg/l of Ca and 1.5 mg/l of
Mg.
Example 4 (according to the invention)
The product obtained according to Example 2 having a mean
particle diameter of 95 ~m was further comminuted by
m~nS of a fluid-bed opposed jet mill with an integrated
mechanical classifier device. Depending on the set speed
of rotation of the classifier, an attrition-free sodium
disilicate having a mean particle diameter of 2 to 15 ~m
was obtained.
Example 5 (according to the invention)
The product obtained according to Example 2 was further
comminuted by means of a porcelain-lined ball mill filled
with corlln~um balls. An attrition-free sodium disilicate
having a mean particle diameter of 5 to 14 ~m, depending
on the grinding time, was obtained.
ple 6 (according to the invention)
The product obtained according to Example 2 was processed
in a roll compactor having a pressing force of the
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compacting rolls of 30 kN/cm of roll width with subse-
quent comminlltion of the flakes in a screen granulator to
give dust-free granules having a mean particle diameter
of 900 ~m, a bulk density of 870 g/l and a high abrasion
resistance.
For the determ;n~tion of the abrasion resis-
tance, 50 g of granules are treated in a roll-
ing ball mill (length: 10 cm; diameter:
11.5 cm; 8 steel balls of 2 cm diameter) for 5
minutes at a speed of rotation of 100 min~l.
After the abrasion test had bee~ carried out, the mean
particle diameter was still 720 ~m, which corresponds to
a decrease of 20~.
Example 7 (according to the invention)
Example 2 was repeated with the modification that the
exit gas from the rotary kiln was extracted at two
points, namely, apart from the region where the amorphous
sodium disilicate having a water content of 19.4% by
weight is introduced, additionally at a point in the
rotary kiln which was at a distance of about 2 m from the
said introduction region in the direction of the rotary
tube axis. The two exit gas streams were combined and the
solids contained therein were precipitated by means of a
heat-resistant hose filter. The precipitated solids were
re-introduced into the rotary kiln together with the
amorphous sodium disilicate having a water content of
19.4~ by weight, so that no sodium disilicate was lost.
As a result, the throughput of the rotary kiln rose to 70
kg/hour, but nevertheless there was no sticky material in
the interior of the rotary kiln.
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Example 8 (comparison example)
Example 2 was repeated with the modification that the hot
gas entering at the top of the hot-air spray tower had a
temperature of 330C. The temperature of the silicate/gas
mixture leaving the spray tower was 140C. The sodium
disilicate precipitated on the hose filter had a bulk
density of 250 g/l, a water content (determined as the
loss on ignition at 700C) of 17.9% by weight and a mean
particle diameter of 60 ~m. This sodium disilicate was
very dusty.
Example 9 (comparison example)
Example 2 was repeated with the modification that the
rotary kiln was insulated only in such a way that, at a
temperature of 490C in the interior of the rotary kiln,
a m~;mllm temperature of 150C occurred on its outer
skin. As a result, large areas of sticking material
formed on the inner wall of the rotary kiln, which fre-
quently had to be knocked off mech~n;cally. From the
rotary kiln, a product was discharged, some of which had
the size of footballs and was very difficult to comminute
by the mechAnic~l crusher.
Example 10 (comparison example)
Example 2 was repeated with the modification that the
sodium disilicate comminuted by means of the mechanical
crusher was ground to a mean particle diameter of 83 ~m,
using an impact disk mill at 10,000 min~l. ~he ground
product had a gray tinge and showed an iron content of
0.02% by weight.
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