CA2024966C - Process for producing sodium silicates - Google Patents
Process for producing sodium silicatesInfo
- Publication number
- CA2024966C CA2024966C CA002024966A CA2024966A CA2024966C CA 2024966 C CA2024966 C CA 2024966C CA 002024966 A CA002024966 A CA 002024966A CA 2024966 A CA2024966 A CA 2024966A CA 2024966 C CA2024966 C CA 2024966C
- Authority
- CA
- Canada
- Prior art keywords
- sodium silicate
- rotary kiln
- weight
- amorphous
- mill
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
- Expired - Fee Related
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Classifications
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B33/00—Silicon; Compounds thereof
- C01B33/20—Silicates
- C01B33/32—Alkali metal silicates
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B33/00—Silicon; Compounds thereof
- C01B33/20—Silicates
- C01B33/32—Alkali metal silicates
- C01B33/325—After-treatment, e.g. purification or stabilisation of solutions, granulation; Dissolution; Obtaining solid silicate, e.g. from a solution by spray-drying, flashing off water or adding a coagulant
-
- C—CHEMISTRY; METALLURGY
- C01—INORGANIC CHEMISTRY
- C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
- C01B33/00—Silicon; Compounds thereof
- C01B33/20—Silicates
- C01B33/36—Silicates having base-exchange properties but not having molecular sieve properties
- C01B33/38—Layered base-exchange silicates, e.g. clays, micas or alkali metal silicates of kenyaite or magadiite type
Abstract
For producing crystalline sodium silicates having a layer structure, an SiO2/Na2O molar ratio of (1.9 to 2.1) : 1 and a water content of less than 0.3% by weight from a waterglass solution containing at least 20% by weight of solids, the waterglass solution is obtained by reacting quartz sand with sodium hydroxide solution at an SiO2/Na2O
molar ratio of (2.0 to 2.3) : 1 at temperatures of 180 to 240°C and pressures of 10 to 30 bar. This waterglass solution is treated in a spray-drying zone with hot air at 200 to 300°C 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 130°C, to form a pulverulent amor-phous sodium silicate having a water content (determined as the loss on ignition at 700°C) of 15 to 23% by weight and a bulk density of more than 300 g/l. The pulverulent, amorphous, water-containing sodium silicate is introduced into an obliquely arranged rotary kiln fitted with devices for moving solids and treated therein with flue gas in countercurrent at temperatures from more than 500 up to 850°C for 1 to 60 minutes to form crystalline soidium silicate. The rotary kiln is here insulated in such a way that its outside wall temperature is less than 60°C.
Finally, the crystalline sodium silicate emerging from the rotary kiln is comminuted by means of a mechanical crusher to grain sizes of 0.1 to 12 mm.
molar ratio of (2.0 to 2.3) : 1 at temperatures of 180 to 240°C and pressures of 10 to 30 bar. This waterglass solution is treated in a spray-drying zone with hot air at 200 to 300°C 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 130°C, to form a pulverulent amor-phous sodium silicate having a water content (determined as the loss on ignition at 700°C) of 15 to 23% by weight and a bulk density of more than 300 g/l. The pulverulent, amorphous, water-containing sodium silicate is introduced into an obliquely arranged rotary kiln fitted with devices for moving solids and treated therein with flue gas in countercurrent at temperatures from more than 500 up to 850°C for 1 to 60 minutes to form crystalline soidium silicate. The rotary kiln is here insulated in such a way that its outside wall temperature is less than 60°C.
Finally, the crystalline sodium silicate emerging from the rotary kiln is comminuted by means of a mechanical crusher to grain sizes of 0.1 to 12 mm.
Description
r~
,` , 2024gG6 _ I ~ HOE 89/H 027 K
The present invention relates to a process for producing crystalline sodium silicates having a layer structure, an SiO2/NazO molar ratio of (1.9 to 2.1) : 1 and a water content of less than 0.3~ by weight from a waterglass solution cont~i n ing at least 20~ by weight of solids.
From US Patent 3,471 r 253, it is known to obtain a water-glass solution by introducing 42% 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 : l.
Crystalline, anhydrous sodium silicates having a layer structure and an SiO2/Na2O molar ratio of (l.g 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% by weight in a spray-drying zone to form a water-cont~ining amor-phous sodium silicate, the exit gas flowing out of the spray-drying zone having a temperature of at least 140C.
The water-cont~in;ng 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% by weight of recycle material, which was obt~;neA by mechanical cnmminlltion of crystalline 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 throughput of material, requires a rotary tube of greater - 2~2~966 dimensions. Finally, due to the use of recycle material at an SiO2/Na2O molar ratio of 2 : 1, a high proportion of the high-temperature modification of sodium disilicate (~-Na2Si2-O5) is formed but it is not the high-temperature modification which is desired, but the ~-modification because of its better builder properties.
According to the invention, the said disadvantages in the production of crystalline sodium silicates having a layer structure from a waterglass solution cont~;n;ng at least 20~ by weight of solids are overcome by a) obt~ining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO2/Na2O
molar ratio of (2.0 to 2.3) : l 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 lO 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 (determined as the loss on ignition at 700~C) o~ 15 to 23% by weight and a bulk density of more than 300 g/l, c) introducing the pulverulent, amorphous, water-cont~;n;ng sodium silicate into an obliquely ar-ranged rotary kiln fitted with devices for moving solids and treating it therein with flue gas in counter-current at temperatures from more than 500 up to 850C for l to 60 minutes to form crystalline sodium silicate, the rotary kiln being insulated in such a way that its outside wall temperature is less than 60C, and d) c~~;nllting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical 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 - . 2024966 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;
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 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, water-cont~; n i ng sodium silicate destined to be introduced into the rotary kiln;
gg) feeding the ground crystalline sodium silicate to a roll compactor, 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.
Crystalline sodium silicates are suitable as reinforcing fillers in natural and synthetic rubber. They also behave as ion exchangers and can therefore be used as a seques-trant.
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 20~96~
. - 4 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~e~cy 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 low-speed mechanical mill (for 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 cont~min~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-con~ining exit gas in the central region of the rotary tube and in the region of its charging end, because 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~ining sodium silicate is charged.
Using the process according to the invention, an abrasion-resistant granulated product, which very quickly disintegrates in water, is obt~ine~ by compacting.
If a waterglass solution having an SiO2/Na2O molar ratio of (2.0 to 2.1) : 1 is used in ~he process according to the invention, a well crystallized sodium disilicate which is m~inly in the form of the ~-modification, has a layer structure, is free of SiO2 and has a lime-combining capacity of at least 80 mg of Ca/g at 20C is - 202~9~6 obt~;ne~ in the flue gas treatment in the rotary kiln at temperatures from 600 to 800C.
Example 1 (according to the state of the art) In a hot-air spray tower (exit gas temperature: 145C), amorphous sodium disilicate having a water content (determi ned as the loss on ignition at 700C) of 19% and a bulk density of 220 g/l was produced from a waterglass solution having a solids content of 45~.
60 kg/hour of amorphous sodium disilicate and 15 kg/h of a recycle material, which had been obtained by co~in~l-tion 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 cryst~lline sodium disilicate was largely pulverulent and had a li~? combining capacity of 74 mg of Ca/g.
Example 2 (according to the invention) Sand (99% by weight of SiO2; grain size: 90% < 0.5 mm) and 50% by weight sodium hydroxide solution 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 in~ecting steam (16 bar) and held for 60 minutes at this te~r~rature. 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 SiOa/Na20 molar ratio of 2.04 : 1 was obtained. The solids content was adjusted to 50% by dilution with water.
The waterglass solution was sprayed in a hot-air spray tower which was fitted with a disk atomiæer and which was heated via a gas-fired combustion chamber and connected to a pneumatically cle~n;ng 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 fi~ter had, at a low dusting t~n~cy, a bulk density of 480 g/l, an iron content of 0.01~ by weight, an SiO2/Na20 ratio of 2.04:1 and a 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 ~acket in such a way that, at a temperature of 730C in the interior of the rotary kiln, a ~imllm temperature of 54C 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 crys-talline sodium disilicate (Na2Si205 having a layer struc-ture) leaving the rotary kiln and showing a water content of 0.1% by weight (determined as the loss on ignition at 700C) was comminllted by means of a mechanical crusher to a grain size of less than 6 mm and, af~er intermediate cooling, ground on a disk mill (diameter: 30 cm) at 400 min~l to a mean particle diameter of 110 ~m, the iron content of the ground product r~ining identical to that of the amorphous sodium disilicate.
2 0 2 ~ 9 ~ ~
_ - 7 -The exit gas from the rotary kiln was extracted only in the region where the amorphous sodium disilicate was introduced, and fed to a scrubbing tower. 5 kg of sodium disilicate per hour were discharged with the exit gas.
Example 3 (according to the invention) The product obtained according to Example 2 having a mean particle diameter of 110 ~m was further comminuted by means 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 and a water content of 0.18% by weight was obtained , the layer structure r~m~in;ng unchanged.
Example 4 (according to the invention) The product obt~in~ according to Example 2 was further comminuted by means of a porcelain-lined ball mill filled with corundum balls. An attrition-free sodium disilicate having a mean particle diameter o~ 5 to 14 ~m, depen~i ng on the grinding time, was obtained , the layer structure re~in;ng unchanged.
Example 5 (according to the invention) The product obt~in~ according to Example 2 was processed in a roll compactor having a pressing force of the compacting rolls of 30 kN/cm of roll width with subse-quent co~;n1ltion of the flakes in a screen granulator to give dust-free granules having a mean particle diameter of 750 ~m, a bulk density of 820 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: lO cm; diameter:
11.5 cm; 8 steel balls of 2 cm diameter) for 5 . 20~4966 ~ 8 minutes at a speed of rotation of 100 min~l.
After the abrasion test had been carried out, the mean particle diameter was still 585 ~m, which corresponds to a decrease of 22%.
Example 6 (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 was 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 m~n~ of a heat-resistant hose filter. The precipitated solids were re-introduced into the rotary kiln together with the amorphous sodium disilicate, 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.
Example 7 (comparison example) Example 2 was repeated with ~he 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 loss on ignition at 700C of 17.9%
by weight and a mean particle diameter of 60 ~m. ~his sodium disilicate was very dusty.
Example 8 (comparison example) Example 2 was repeated with the modification that a waterglass solution having an SiO2/NazO molar ratio of 20~4~S
, g 2.15 : 1 was prepared and sprayed in the hot-air spray tower to give an amorphous sodium disilicate having an SiO2/Na2O ratio of 2.15 : 1. In the rotary kiln at 730C, this gave a crystalline sodium disilicate which showed, in the x-ray diagram, the lines of the undesired by-product cristohAlite (SiO2) which is responsible for a reduction in the lime-combining capacity and leeds to a deterioration in the builder properties.
RxAmrle 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 710C in the interior of the rotary kiln, 8 m-ximllm temperature of 205C 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 mechanically. From the rotary kiln, a very hard, poorly crystallized product was discharged, some of which had the size of footballs and was very difficult to comminute by the mechanical crusher.
Example 10 (comparison example) RxAm~le 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 98 ~m, using an impact disk mill at 10,000 min~l. The ground product had a gray tinge and showed an iron content of 0.025% by weight.
RxAmrle 11 (comparison example) RxAmple 5 was repeated with the modification that the pressing force of the compacting rolls was only 15 kN/cm of roll width. The resulting granules had a mean particle diameter of 680 ~m and a~ensity of 790 g/l. After the abrasion test had been carried out, the mean particle - 202~6~
. - 10 -diameter was then only 265 ~m, which correspond to a decrease of 61%. The granules were soft, and some of them already disintegrated into smaller agglomerates on packing.
The lime-combining capacity, indicated in the table which follows, of the sodium silicates, obtA; n~ in the ex-amples, having a layer structure was det~rmine~ by the following procedure:
The CaCl2 solution (corresponding to 300 mg of CaO) was added to 1 1 of distilled water, whereby a water with 30 German hardness was obtA i n~ .
1 g of the crystalline sodium silicate obtained in the examples and 0 to 6 ml of a one-molar glycine solution (obtAin~ from 75.1 g of glycine and 58.4 g of NaCl, dissolved with water up to 1 1) were added to 1 1 of the above water, the temperature of which had been ad~usted either to 20 or to 60C, whereupon a pH of 10.4 established itself. The suspension was stirred for 30 minutes at the selected temperature (20 or 60C~, during which the pH remained stable. Finally, the solu-tion was filtered and the calcium remAining in solution in the filtrate was determi n~ by com-plexometry. The li~ combining capacity was determined by forming the difference with the original contents.
. . , . , _ . . .
-TABLE
Lime-combining capacity of sodium silicate at pH 10.4 (in mg Ca/g Na2Si2O5) Example at 20C at 60C
2, 3, 4, 5, 10, 11 82 132
,` , 2024gG6 _ I ~ HOE 89/H 027 K
The present invention relates to a process for producing crystalline sodium silicates having a layer structure, an SiO2/NazO molar ratio of (1.9 to 2.1) : 1 and a water content of less than 0.3~ by weight from a waterglass solution cont~i n ing at least 20~ by weight of solids.
From US Patent 3,471 r 253, it is known to obtain a water-glass solution by introducing 42% 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 : l.
Crystalline, anhydrous sodium silicates having a layer structure and an SiO2/Na2O molar ratio of (l.g 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% by weight in a spray-drying zone to form a water-cont~ining amor-phous sodium silicate, the exit gas flowing out of the spray-drying zone having a temperature of at least 140C.
The water-cont~in;ng 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% by weight of recycle material, which was obt~;neA by mechanical cnmminlltion of crystalline 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 throughput of material, requires a rotary tube of greater - 2~2~966 dimensions. Finally, due to the use of recycle material at an SiO2/Na2O molar ratio of 2 : 1, a high proportion of the high-temperature modification of sodium disilicate (~-Na2Si2-O5) is formed but it is not the high-temperature modification which is desired, but the ~-modification because of its better builder properties.
According to the invention, the said disadvantages in the production of crystalline sodium silicates having a layer structure from a waterglass solution cont~;n;ng at least 20~ by weight of solids are overcome by a) obt~ining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO2/Na2O
molar ratio of (2.0 to 2.3) : l 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 lO 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 (determined as the loss on ignition at 700~C) o~ 15 to 23% by weight and a bulk density of more than 300 g/l, c) introducing the pulverulent, amorphous, water-cont~;n;ng sodium silicate into an obliquely ar-ranged rotary kiln fitted with devices for moving solids and treating it therein with flue gas in counter-current at temperatures from more than 500 up to 850C for l to 60 minutes to form crystalline sodium silicate, the rotary kiln being insulated in such a way that its outside wall temperature is less than 60C, and d) c~~;nllting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical 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 - . 2024966 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;
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 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, water-cont~; n i ng sodium silicate destined to be introduced into the rotary kiln;
gg) feeding the ground crystalline sodium silicate to a roll compactor, 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.
Crystalline sodium silicates are suitable as reinforcing fillers in natural and synthetic rubber. They also behave as ion exchangers and can therefore be used as a seques-trant.
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 20~96~
. - 4 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~e~cy 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 low-speed mechanical mill (for 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 cont~min~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-con~ining exit gas in the central region of the rotary tube and in the region of its charging end, because 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~ining sodium silicate is charged.
Using the process according to the invention, an abrasion-resistant granulated product, which very quickly disintegrates in water, is obt~ine~ by compacting.
If a waterglass solution having an SiO2/Na2O molar ratio of (2.0 to 2.1) : 1 is used in ~he process according to the invention, a well crystallized sodium disilicate which is m~inly in the form of the ~-modification, has a layer structure, is free of SiO2 and has a lime-combining capacity of at least 80 mg of Ca/g at 20C is - 202~9~6 obt~;ne~ in the flue gas treatment in the rotary kiln at temperatures from 600 to 800C.
Example 1 (according to the state of the art) In a hot-air spray tower (exit gas temperature: 145C), amorphous sodium disilicate having a water content (determi ned as the loss on ignition at 700C) of 19% and a bulk density of 220 g/l was produced from a waterglass solution having a solids content of 45~.
60 kg/hour of amorphous sodium disilicate and 15 kg/h of a recycle material, which had been obtained by co~in~l-tion 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 cryst~lline sodium disilicate was largely pulverulent and had a li~? combining capacity of 74 mg of Ca/g.
Example 2 (according to the invention) Sand (99% by weight of SiO2; grain size: 90% < 0.5 mm) and 50% by weight sodium hydroxide solution 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 in~ecting steam (16 bar) and held for 60 minutes at this te~r~rature. 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 SiOa/Na20 molar ratio of 2.04 : 1 was obtained. The solids content was adjusted to 50% by dilution with water.
The waterglass solution was sprayed in a hot-air spray tower which was fitted with a disk atomiæer and which was heated via a gas-fired combustion chamber and connected to a pneumatically cle~n;ng 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 fi~ter had, at a low dusting t~n~cy, a bulk density of 480 g/l, an iron content of 0.01~ by weight, an SiO2/Na20 ratio of 2.04:1 and a 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 ~acket in such a way that, at a temperature of 730C in the interior of the rotary kiln, a ~imllm temperature of 54C 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 crys-talline sodium disilicate (Na2Si205 having a layer struc-ture) leaving the rotary kiln and showing a water content of 0.1% by weight (determined as the loss on ignition at 700C) was comminllted by means of a mechanical crusher to a grain size of less than 6 mm and, af~er intermediate cooling, ground on a disk mill (diameter: 30 cm) at 400 min~l to a mean particle diameter of 110 ~m, the iron content of the ground product r~ining identical to that of the amorphous sodium disilicate.
2 0 2 ~ 9 ~ ~
_ - 7 -The exit gas from the rotary kiln was extracted only in the region where the amorphous sodium disilicate was introduced, and fed to a scrubbing tower. 5 kg of sodium disilicate per hour were discharged with the exit gas.
Example 3 (according to the invention) The product obtained according to Example 2 having a mean particle diameter of 110 ~m was further comminuted by means 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 and a water content of 0.18% by weight was obtained , the layer structure r~m~in;ng unchanged.
Example 4 (according to the invention) The product obt~in~ according to Example 2 was further comminuted by means of a porcelain-lined ball mill filled with corundum balls. An attrition-free sodium disilicate having a mean particle diameter o~ 5 to 14 ~m, depen~i ng on the grinding time, was obtained , the layer structure re~in;ng unchanged.
Example 5 (according to the invention) The product obt~in~ according to Example 2 was processed in a roll compactor having a pressing force of the compacting rolls of 30 kN/cm of roll width with subse-quent co~;n1ltion of the flakes in a screen granulator to give dust-free granules having a mean particle diameter of 750 ~m, a bulk density of 820 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: lO cm; diameter:
11.5 cm; 8 steel balls of 2 cm diameter) for 5 . 20~4966 ~ 8 minutes at a speed of rotation of 100 min~l.
After the abrasion test had been carried out, the mean particle diameter was still 585 ~m, which corresponds to a decrease of 22%.
Example 6 (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 was 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 m~n~ of a heat-resistant hose filter. The precipitated solids were re-introduced into the rotary kiln together with the amorphous sodium disilicate, 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.
Example 7 (comparison example) Example 2 was repeated with ~he 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 loss on ignition at 700C of 17.9%
by weight and a mean particle diameter of 60 ~m. ~his sodium disilicate was very dusty.
Example 8 (comparison example) Example 2 was repeated with the modification that a waterglass solution having an SiO2/NazO molar ratio of 20~4~S
, g 2.15 : 1 was prepared and sprayed in the hot-air spray tower to give an amorphous sodium disilicate having an SiO2/Na2O ratio of 2.15 : 1. In the rotary kiln at 730C, this gave a crystalline sodium disilicate which showed, in the x-ray diagram, the lines of the undesired by-product cristohAlite (SiO2) which is responsible for a reduction in the lime-combining capacity and leeds to a deterioration in the builder properties.
RxAmrle 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 710C in the interior of the rotary kiln, 8 m-ximllm temperature of 205C 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 mechanically. From the rotary kiln, a very hard, poorly crystallized product was discharged, some of which had the size of footballs and was very difficult to comminute by the mechanical crusher.
Example 10 (comparison example) RxAm~le 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 98 ~m, using an impact disk mill at 10,000 min~l. The ground product had a gray tinge and showed an iron content of 0.025% by weight.
RxAmrle 11 (comparison example) RxAmple 5 was repeated with the modification that the pressing force of the compacting rolls was only 15 kN/cm of roll width. The resulting granules had a mean particle diameter of 680 ~m and a~ensity of 790 g/l. After the abrasion test had been carried out, the mean particle - 202~6~
. - 10 -diameter was then only 265 ~m, which correspond to a decrease of 61%. The granules were soft, and some of them already disintegrated into smaller agglomerates on packing.
The lime-combining capacity, indicated in the table which follows, of the sodium silicates, obtA; n~ in the ex-amples, having a layer structure was det~rmine~ by the following procedure:
The CaCl2 solution (corresponding to 300 mg of CaO) was added to 1 1 of distilled water, whereby a water with 30 German hardness was obtA i n~ .
1 g of the crystalline sodium silicate obtained in the examples and 0 to 6 ml of a one-molar glycine solution (obtAin~ from 75.1 g of glycine and 58.4 g of NaCl, dissolved with water up to 1 1) were added to 1 1 of the above water, the temperature of which had been ad~usted either to 20 or to 60C, whereupon a pH of 10.4 established itself. The suspension was stirred for 30 minutes at the selected temperature (20 or 60C~, during which the pH remained stable. Finally, the solu-tion was filtered and the calcium remAining in solution in the filtrate was determi n~ by com-plexometry. The li~ combining capacity was determined by forming the difference with the original contents.
. . , . , _ . . .
-TABLE
Lime-combining capacity of sodium silicate at pH 10.4 (in mg Ca/g Na2Si2O5) Example at 20C at 60C
2, 3, 4, 5, 10, 11 82 132
Claims (9)
1. A process for producing crystalline sodium silicates having a layer structure, an SiO2/Na2O molar ratio of (1.9 to 2.1) : 1 and a water content of less than 0.3% by weight from a waterglass solution containing at least 20%
by weight of solids, which comprises a) obtaining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO2/Na2O
molar ratio of (2.0 to 2.3) : 1 at temperatures from 180 to 240°C and pressures from 10 to 30 bar, b) treating the waterglass solution in a spray-drying zone with hot air at 200 to 300°C 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 130°C, to form a pulverulent amorphous sodium sili-cate having a water content (determined as the loss on ignition at 700°C) of 15 to 23% by weight and a bulk density of more than 300 g/l, c) introducing the pulverulent, amorphous, water-containing sodium silicate into an obliquely ar-ranged rotary kiln fitted with devices for moving solids and treating it therein with flue gas in counter-current at temperatures from more than 500 to 850°C for 1 to 60 minutes to form crystalline sodium silicate, the rotary kiln being insulated in such a way that its outside wall temperature is less than 60°C, and d) comminuting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical crusher to grain sizes of 0.1 to 12 mm.
by weight of solids, which comprises a) obtaining the waterglass solution by reacting quartz sand with sodium hydroxide solution in an SiO2/Na2O
molar ratio of (2.0 to 2.3) : 1 at temperatures from 180 to 240°C and pressures from 10 to 30 bar, b) treating the waterglass solution in a spray-drying zone with hot air at 200 to 300°C 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 130°C, to form a pulverulent amorphous sodium sili-cate having a water content (determined as the loss on ignition at 700°C) of 15 to 23% by weight and a bulk density of more than 300 g/l, c) introducing the pulverulent, amorphous, water-containing sodium silicate into an obliquely ar-ranged rotary kiln fitted with devices for moving solids and treating it therein with flue gas in counter-current at temperatures from more than 500 to 850°C for 1 to 60 minutes to form crystalline sodium silicate, the rotary kiln being insulated in such a way that its outside wall temperature is less than 60°C, and d) comminuting the crystalline sodium silicate emerging from the rotary kiln by means of a mechanical crusher to grain sizes of 0.1 to 12 mm.
2. The process as claimed in claim 1, wherein the comminuted sodium silicate is ground by means of a mill to grain sizes of 2 to 400 µm.
3. The process as claimed in claim 2, wherein a mechanical mill running at a circumferential speed of 0.5 to 60 m/s is used.
4. The process as claimed in claim 2, wherein an air jet mill is used.
5. The process as claimed in claim 2, wherein a ceramically lined ball mill is used.
6. The process as claimed in claim 2, wherein a ceramically lined vibratory mill is used.
7. The process as claimed in claim 1, wherein the exit gas from the rotary kiln is extracted in the central region thereof and in the region of the end where the pulveru-lent amorphous sodium silicate is introduced, and pur-ified by means of a dry dust filter, the sodium silicate taken from the dry dust filter being quasi-continuously admixed to the pulverulent, amorphous, water-containing sodium silicate destined to be introduced into the rotary kiln.
8. The process as claimed in claim 1, wherein the ground anhydrous sodium silicate is fed to a roll compactor, 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.
9. The process as claimed in claim 8, wherein the compact pieces are, after pre-comminution by forcing them through screens, processed to give granules having a bulk density of 700 to 1,000 g/l.
Applications Claiming Priority (4)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| DEP3935464.4 | 1989-10-25 | ||
| DE3935464 | 1989-10-25 | ||
| DEP4004624.9 | 1990-02-15 | ||
| DE4004624 | 1990-02-15 |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| CA2024966A1 CA2024966A1 (en) | 1991-04-26 |
| CA2024966C true CA2024966C (en) | 1995-07-18 |
Family
ID=25886411
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA002024966A Expired - Fee Related CA2024966C (en) | 1989-10-25 | 1990-09-10 | Process for producing sodium silicates |
Country Status (18)
| Country | Link |
|---|---|
| EP (1) | EP0425428B1 (en) |
| JP (1) | JPH0669890B2 (en) |
| KR (1) | KR0143228B1 (en) |
| AT (2) | ATE98609T1 (en) |
| BR (1) | BR9005378A (en) |
| CA (1) | CA2024966C (en) |
| DE (3) | DE59003875D1 (en) |
| DK (1) | DK0425428T3 (en) |
| ES (1) | ES2047898T3 (en) |
| FI (1) | FI905209A0 (en) |
| HR (1) | HRP921197B1 (en) |
| LT (2) | LTIP1445A (en) |
| LV (1) | LV10763B (en) |
| NO (1) | NO904603L (en) |
| PT (1) | PT95665A (en) |
| RU (1) | RU2032619C1 (en) |
| SI (1) | SI9012001A (en) |
| TR (1) | TR25165A (en) |
Families Citing this family (29)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB8904007D0 (en) * | 1989-02-22 | 1989-04-05 | Procter & Gamble | Stabilized,bleach containing,liquid detergent compositions |
| DE4107230C2 (en) * | 1991-03-07 | 1995-04-06 | Hoechst Ag | Process for the production of sodium silicates |
| DE4142711A1 (en) * | 1991-12-21 | 1993-06-24 | Hoechst Ag | METHOD FOR PRODUCING CRYSTALLINE SODIUM DISILICATES |
| DE4329394B4 (en) * | 1993-09-01 | 2006-11-02 | Henkel Kgaa | Builder component for detergents or cleaners |
| DE4330868A1 (en) * | 1993-09-11 | 1995-03-16 | Hoechst Ag | Process for preparing granular sodium silicate |
| DE4400024A1 (en) * | 1994-01-03 | 1995-07-06 | Henkel Kgaa | Silicate builders and their use in detergents and cleaning agents as well as multi-component mixtures for use in this field |
| DE4406592A1 (en) * | 1994-03-01 | 1995-09-07 | Henkel Kgaa | Improved multi-substance mixtures based on water-soluble alkali silicate compounds and their use, in particular for use as builders in detergents and cleaners |
| DE19515072A1 (en) * | 1995-04-28 | 1996-10-31 | Cognis Bio Umwelt | Detergent containing cellulase |
| DE19525197A1 (en) * | 1995-07-11 | 1997-01-16 | Hoechst Ag | Granular detergent builder |
| DE19537957A1 (en) * | 1995-10-12 | 1997-04-17 | Hoechst Ag | Process for the production of highly exchangeable sodium disilicate |
| DE19545729A1 (en) | 1995-12-08 | 1997-06-12 | Henkel Kgaa | Bleach and detergent with an enzymatic bleaching system |
| DE19600018A1 (en) | 1996-01-03 | 1997-07-10 | Henkel Kgaa | Detergent with certain oxidized oligosaccharides |
| DE19605688A1 (en) * | 1996-02-16 | 1997-08-21 | Henkel Kgaa | Transition metal complexes as activators for peroxygen compounds |
| US6235695B1 (en) | 1996-04-01 | 2001-05-22 | Henkel Kommanditgesellschaft Auf Aktien | Cleaning agent with oligoammine activator complexes for peroxide compounds |
| DE19620411A1 (en) | 1996-04-01 | 1997-10-02 | Henkel Kgaa | Transition metal amine complexes as activators for peroxygen compounds |
| DE19616693A1 (en) * | 1996-04-26 | 1997-11-06 | Henkel Kgaa | Enol esters as bleach activators for detergents and cleaning agents |
| DE19707448C2 (en) * | 1997-02-25 | 1998-11-26 | Clariant Gmbh | Process for the production of crystalline sodium silicates |
| DE19707449C1 (en) * | 1997-02-25 | 1998-04-09 | Clariant Gmbh | Layered sodium silicate preparation with high delta-phase content |
| DE19914811A1 (en) | 1999-03-31 | 2000-10-05 | Henkel Kgaa | Detergent compositions containing a bleaching agent include a combination of a cyanomethyl ammonium salt bleach activator and an enzyme |
| DE19943551A1 (en) | 1999-09-11 | 2001-03-15 | Clariant Gmbh | Poorly soluble alkali silicate |
| DE19943550A1 (en) * | 1999-09-11 | 2001-03-15 | Clariant Gmbh | Highly alkaline crystalline sodium silicate |
| DE10058645A1 (en) | 2000-11-25 | 2002-05-29 | Clariant Gmbh | Use of cyclic sugar ketones as catalysts for peroxygen compounds |
| DE10102248A1 (en) | 2001-01-19 | 2002-07-25 | Clariant Gmbh | Use of transition metal complexes with oxime ligands as bleach catalysts |
| DE102005026544A1 (en) | 2005-06-08 | 2006-12-14 | Henkel Kgaa | Reinforcement of cleaning performance of detergents by polymer |
| ES2425017B1 (en) * | 2010-10-26 | 2014-09-30 | Antonio Arnau Villanova | GRANULATION BY AGLOMERATION OF CERAMIC COMPOSITIONS MOLDED IN DRY PHASE |
| DE102011080235A1 (en) * | 2011-08-01 | 2013-02-07 | Wacker Chemie Ag | Process for the preparation of dispersion powders |
| CN114073915A (en) * | 2020-08-21 | 2022-02-22 | 博特化工装置股份公司 | Device for producing alkali metal hydroxide particles |
| CN113753906A (en) * | 2021-09-16 | 2021-12-07 | 中国建设基础设施有限公司 | Wet grinding preparation method of water glass excitant for geopolymer |
| CN115154053B (en) * | 2022-07-08 | 2023-03-10 | 福建大山纸业有限公司 | Paper diaper buckling device and corresponding cotton core independent forming process thereof |
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| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US1517891A (en) * | 1922-09-01 | 1924-12-02 | Ind Waste Products Corp | Powdered sodium silicate and process of preparing the same |
| US3471253A (en) | 1966-05-24 | 1969-10-07 | Cowles Chem Co | Process for producing sodium metasilicate pentahydrate |
| US3838192A (en) * | 1971-10-28 | 1974-09-24 | Huber Corp J M | Production of alkali metal polysilicates |
| DE3423945A1 (en) * | 1984-06-29 | 1986-01-09 | Henkel KGaA, 4000 Düsseldorf | METHOD AND DEVICE FOR THE CONTINUOUS HYDROTHERMAL PRODUCTION OF SODIUM SILICATE SOLUTIONS |
| RO95539B1 (en) * | 1986-09-04 | 1988-10-01 | Institutul De Cercetari Metalurgice | Process for preparing sodium silicate powder |
| DE3718350A1 (en) | 1987-06-01 | 1988-12-22 | Hoechst Ag | METHOD FOR PRODUCING CRYSTALLINE SODIUM SILICATES WITH LAYER STRUCTURE |
-
1990
- 1990-09-10 CA CA002024966A patent/CA2024966C/en not_active Expired - Fee Related
- 1990-10-08 AT AT90710028T patent/ATE98609T1/en not_active IP Right Cessation
- 1990-10-08 DK DK90710028.3T patent/DK0425428T3/en active
- 1990-10-08 AT AT90710027T patent/ATE98608T1/en not_active IP Right Cessation
- 1990-10-08 DE DE90710028T patent/DE59003875D1/en not_active Expired - Fee Related
- 1990-10-08 ES ES90710028T patent/ES2047898T3/en not_active Expired - Lifetime
- 1990-10-08 DE DE4031848A patent/DE4031848A1/en not_active Withdrawn
- 1990-10-08 DE DE90710027T patent/DE59003874D1/en not_active Expired - Fee Related
- 1990-10-08 EP EP90710028A patent/EP0425428B1/en not_active Expired - Lifetime
- 1990-10-23 TR TR90/1035A patent/TR25165A/en unknown
- 1990-10-23 FI FI905209A patent/FI905209A0/en not_active IP Right Cessation
- 1990-10-23 KR KR1019900016930A patent/KR0143228B1/en not_active IP Right Cessation
- 1990-10-24 RU SU904831290A patent/RU2032619C1/en active
- 1990-10-24 PT PT95665A patent/PT95665A/en not_active Application Discontinuation
- 1990-10-24 JP JP2284527A patent/JPH0669890B2/en not_active Expired - Fee Related
- 1990-10-24 BR BR909005378A patent/BR9005378A/en not_active IP Right Cessation
- 1990-10-24 NO NO90904603A patent/NO904603L/en unknown
- 1990-10-24 SI SI9012001A patent/SI9012001A/en unknown
-
1992
- 1992-11-06 HR HRP-2001/90A patent/HRP921197B1/en not_active IP Right Cessation
-
1993
- 1993-05-04 LV LVP-93-277A patent/LV10763B/en unknown
- 1993-11-05 LT LTIP1445A patent/LTIP1445A/en not_active Application Discontinuation
- 1993-11-05 LT LTIP1436A patent/LT3795B/en not_active IP Right Cessation
Also Published As
| Publication number | Publication date |
|---|---|
| ATE98608T1 (en) | 1994-01-15 |
| TR25165A (en) | 1992-11-01 |
| HRP921197B1 (en) | 1998-02-28 |
| LV10763A (en) | 1995-08-20 |
| LT3795B (en) | 1996-03-25 |
| RU2032619C1 (en) | 1995-04-10 |
| NO904603D0 (en) | 1990-10-24 |
| ATE98609T1 (en) | 1994-01-15 |
| CA2024966A1 (en) | 1991-04-26 |
| BR9005378A (en) | 1991-09-17 |
| JPH0669890B2 (en) | 1994-09-07 |
| DK0425428T3 (en) | 1994-02-21 |
| FI905209A0 (en) | 1990-10-23 |
| LTIP1436A (en) | 1995-05-25 |
| JPH03164422A (en) | 1991-07-16 |
| EP0425428A2 (en) | 1991-05-02 |
| SI9012001A (en) | 1997-12-31 |
| ES2047898T3 (en) | 1994-03-01 |
| DE59003875D1 (en) | 1994-01-27 |
| EP0425428B1 (en) | 1993-12-15 |
| KR0143228B1 (en) | 1998-07-15 |
| LTIP1445A (en) | 1995-05-25 |
| DE59003874D1 (en) | 1994-01-27 |
| NO904603L (en) | 1991-04-26 |
| HRP921197A2 (en) | 1995-10-31 |
| LV10763B (en) | 1995-12-20 |
| PT95665A (en) | 1991-09-13 |
| DE4031848A1 (en) | 1991-05-02 |
| KR910007803A (en) | 1991-05-30 |
| EP0425428A3 (en) | 1992-04-29 |
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