SI9012001A - Method for preparation of sodium silicates - Google Patents

Abstract

In order to prepare crystalline sodium silicates having a layered structure, a water content of less than 0.3% by weight and an SiO2 to Na2O molar ratio of (1.9 to 2.1):1 from a water glass solution containing at least 20% by weight of solid, the water glass solution is obtained by reaction of silica sand with sodium hydroxide solution in an SiO2 to Na2O molar ratio of (2.0 to 2.3):1 at temperatures of 180 to 240 DEG C and pressures of 10 to 30 bar. This water glass solution is treated in a spray-drying zone with hot air of 200 to 300 DEG C at a residence time of 10 to 25 s and a temperature of the waste gas leaving the spray-drying zone of 90 to 130 DEG C with formation of a pulverulent amorphous sodium silicate having a water content (determined as loss on ignition at 700 DEG 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 placed in a tilted rotary kiln equipped with means for moving solids and treated therein counter-currently with flue gas at temperatures of more than 500 DEG C to 850 DEG C for 1 to 60 minutes with formation of crystalline sodium silicate. The rotary kiln is insulated here in such a manner that the temperature of its outer wall is less than 60 DEG C. Finally, the crystalline sodium silicate leaving the rotary kiln is comminuted by means of a mechanical crusher to particle sizes of 0.1 to 12 mm.

Description

HOECHST AKTIENGESELLSCHAFT

Process for the preparation of sodium silicates

The present invention relates to a process for the preparation of crystalline sodium silicates with a layered structure with a molar ratio of SiO ₂ to Na ₂ O (1.9 to 2.1): 1 and a water content of less than 0.3% by weight of soluble glass solution with at least 20% by weight of solid.

It is known from US P 3 471 253 that a solution of soluble glass can be obtained by placing 42% by weight of sodium alkali and sand (silica) about 2: 1 in a mixing autoclave and leaving in it for 3 hours at 210 ° C and 16 bar. The hot sodium silicate solution, which is removed after cooling the autoclave content to 85 ° C, contains 57.5% solids after filtration of excess sand and other impurities and shows a SiO ₂ : Na ₂ O ratio of 1.64: 1.

Crystalline anhydrous sodium silicates with a layered structure and a molar ratio of SiO ₂ to Na ₂ O (1.9 to 3.5): 1 are prepared by the method of DE-OS 37 18 350 by treating soluble glass solutions containing solids from 20 to 65% by weight in the spray drying zone to form amorphous sodium silicate containing water, wherein the waste gas exiting the spray drying zone has a temperature of at least 140 ° C. Amorphous sodium silicate containing water is tempered in the annealing zone at 500 to 800 ° C for 1 to 60 minutes in the presence of at least 10% by weight of the return material obtained by mechanical crushing of crystalline sodium silicate previously extracted from annealing zones.

In the process just mentioned, it is disadvantageous to require a large bulk density of 100 to 250 g / l and to be highly dusty due to its low bulk density. Furthermore, the use of return material during tempering is significantly more costly for appliances and requires a larger dimensioned rotary tube due to the greater material flow.

Finally, due to the use of feedback material at the molar ratio between SiO ₂ and Na ₂ O 2: 1, a high proportion of high-temperature modification of sodium disilicate (a-Na ₂ Si ₂ O ₅ ) is achieved, but not due to its high-temperature builder properties, δ -modification.

According to the invention, the disadvantages of the preparation of crystalline sodium silicates with a layered structure from a soluble glass solution of at least 20% by weight of a solid are overcome by such that

a) obtain a solution of soluble glass by the digestion of silica sand with sodium hydroxide in a molar ratio of SiO ₂ : Na ₂ O (2.0 to 2.3): 1 at temperatures from 180 to 240 ° C and pressures from 10 to 30 bar;

b) treat the solution of soluble glass in the spray drying zone in hot air from 200 to 300 ° C at a holding time of 10 to 25 s and a temperature of the waste gas leaving the spray zone, 90 to 130 ° C, to form a powdered amorphous sodium silicate with a content of water (determined as annealing loss at 700 ° C) of 15 to 23% by weight and a bulk density of more than 300 g / l;

c) Put powdery, amorphous sodium silicate containing water into an obliquely rotating tube furnace equipped with solid-state moving devices and counter-flow with a flue gas at temperatures of more than 500 to 850 ° C for 1 to 60 minutes at formation of crystalline sodium silicate, wherein the rotary tube furnace is insulated so that the temperature of its outer wall is less than 60 ° C;

d) the crystalline sodium silicate exiting the rotary tube furnace is crushed by means of a mechanical shredder to a grain size of 0.1 to 12 mm.

The process of the invention may further be optionally further formed by aa) grinding the crushed sodium silicate using a mill on a grain size of 2 to 400 gm;

bb) use a mechanical mill operating at a circumferential speed of 0.5 to 60 m / s;

cc) use an air jet mill;

dd) use a ball mill with a ceramic coating;

ee) use a swing mill with a ceramic coating;

ff) aspirate the waste gas from a rotary tube furnace in its middle region and in the region of its end which requires the introduction of powdered amorphous sodium silicate and purify it by means of a dry dust filter, quasi-continually stirring the sodium silicate collected from it. a dry dust filter, a powdery, amorphous sodium silicate containing water, which is intended to be introduced into a rotary tube furnace;

gg) the ground crystalline sodium silicate is fed into a cylindrical compacting device, which is pressed into compact parts at a cylinder pressure of 200 to 40 kN / cm;

hh) process the compact parts after preliminary crushing by compressing them through sieves into a granulate with a bulk weight of 700 to 1000 g / l.

Crystalline sodium silicates are suitable as reinforcing agents in natural and synthetic rubbers. They further act as ion exchangers and can therefore be used as sequestrants.

In the process according to the invention, due to the low temperature and short retention time of the solution of the soluble glass solution, sodium silicate with a large bulk mass can be handled.

Due to its good insulation, a small heat transfer through the walls of the rotary tube furnace works in the process of the invention against the tendency of sodium silicate to adhere.

The process of the invention requires the use of a slow-motion mechanical mill (e.g., a mill mill, a hammer mill, a hammer mill or a rolling mill) to prevent the wear of the iron from grinding tools.

If a ball mill with a ceramic coating or a pendulum or air jet mill is used in the process according to the invention for the finest products, i.e. with diameters of 6 to 10 μτη, there is also no contamination of sodium silicate due to metal wear.

By simultaneously aspirating the waste gas containing the dust in the middle region of the rotary tube furnace and in the region of its end on the inlet side of the process according to the invention, the dust load in the waste gas is significantly reduced, since the dust is released mainly by adding sodium silicate to the rotary tube and because the gas velocity in the feed area of amorphous sodium silicate containing water decreases.

By the process of the invention, a wear-resistant granulate is obtained by compacting against wear and tear, which breaks down very quickly in water.

When used in the process according to the invention, a solution of soluble glass with a molecular ratio of SiO ₂ to Na ₂ O (2.0 to 2.1): 1 is obtained by treatment in a rotary tube furnace with a flue gas at temperatures from 600 to 800 ° C. well crystallized sodium disilicate with a layered structure mainly found in δ-modification, which is SiO _2-free and exhibits at 20 ° C a lime-binding capacity of at least 80 mg Ca / g.

Example 1 (state of the art)

From a soluble glass solution containing 45% solids, amorphous sodium disilicate, which showed water content (determined as annealing loss at 700 ° C), was made in a hot air spray tower (waste gas temperature: 154 ° C), and bulk density 220 g / l.

A directly fired rotary tube furnace (5 m in length, 78 cm in diameter, 1.2 ° inclination) was added 605cg / h of amorphous sodium disilicate and 15 kg / h of return material at the end of the flame opposite the flame. it was obtained by crushing the product obtained in one of the preceding batches to less than 250 μτη, while the crystalline product was brought out on the flame side. The temperature at the hottest place of the rotary tube furnace was 740 ° C.

No adhesions were formed on the wall of the rotary tube furnace, and the disilicate sodium discharged was far-dusted and showed a lime-binding capacity of 74 mg Ca / g.

Example 2 (Invention)

Sand (99% by weight SiO ₂ ; granularity: 90% <0.5 mm) and 50% by weight - no sodium hydroxide in a molar ratio of SiO ₂ to Na ₂ O was filled into a cylindrical autoclave with a nickel plating mixer. 2.15: 1. The mixture in the autoclave was heated to 200 [deg.] C. while stirring by compressing the steam (16 bar) and maintained at this temperature for 60 minutes. The contents of the autoclave were then decompressed via a evaporation vessel into another vessel and, to separate the insoluble, by the addition of 0.3 wt.% Perlite, as an auxiliary filtering agent, was filtered at 90 ° C via a plate pressure filter. The filtrate was obtained as a clear solution of soluble glass with a molar ratio of SiO ₂ to Na ₂ O 2.04: 1. By dilution with water, the solids content was adjusted to 50%.

In a hot air spray tower equipped with a centrifugal plate sprayer which was heated via a gas-fired combustion chamber and connected to the bag filter by pneumatic cleaning to separate the product, a solution of soluble glass was sprayed, adjusting the combustion chamber by , that the hot gas entering the top of the tower had a temperature of 260 ° C. The amount of soluble glass solution to be sprayed was adjusted so that the temperature of the mixture of silicate and gas leaving the spray tower was 105 ° C. A residence time of 16 seconds was calculated from the volume of the spray tower and the amount of gas flow through the spray tower. In the bag filter, the amorphous sodium disilicate extract eliminated, with a low propensity to dust, a bulk density of 480 g / l, an iron content of 0.01 wt%, a SiO ratio of ₂ : Na ₂ O 2.04: 1 and an annealing loss at 700 ° C. 19.4%; its mean particle diameter was 52 / im.

In Example 1, the rotary tube furnace described was insulated with a multi-layer mineral wool and a sheet sheath so that at a temperature of 730 ° C, the inside surface of the rotary tube furnace was at a maximum temperature of 54 ° C. 60 kg of amorphous sodium disilicate per hour were introduced into this rotary tube furnace, with no pasting. Crystalline sodium disilicate (Na ₂ Si ₂ O ₅ ) with a layered structure leaving a rotary tube furnace and having a water content of 0.1% by weight (determined as an annealing loss at 700 ° C), using a mechanical shredder crushed to less than 6 mm and after intermediate cooling ground on a disc mill (30 cm diameter) at 400 min ¹ to a mean particle diameter of 110 μτη, leaving the iron content of the ground product equal to the iron content of amorphous sodium disilicate.

The exhaust gas of the rotary tube furnace was sucked off only in the intake area for amorphous sodium disilicate and was fed into the wash tower. Waste gas was extracted with 5 kg of sodium disilicate per hour.

Example 3 (Invention)

According to Example 2, the resulting product with a mean particle diameter of 110 μτη was further crushed by means of a fluidized bed antistatic mill with a built-in mechanical-grade preparation. Depending on the set speed of the cluster preparation, sodium disilicate was obtained without wear, with a mean particle diameter of 2 to 15 μτη and a water content of 0.18 wt.%, While maintaining the layered structure unchanged.

Example 4 (Invention)

In Example 2, the resulting product was further crushed using a porcelain-coated ball mill filled with corundum spheres. Disilicate sodium disilicate was obtained with a mean particle diameter of 5 to 14 [mu] m, depending on the duration of milling, while maintaining the layered structure unchanged.

Example 5 (Invention)

In Example 2, the resulting product was rolled into a compact roller assembly with a compression cylinder pressure of 30 kN / cm roll width, and the subsequent crushing of the resulting material in a sieve granulator was transformed into a powder free granule with a mean particle diameter of 750 / im, a bulk mass of 820 g / 1 and high abrasion resistance.

To determine the abrasion resistance, we treated 50 g of granulate in a rolling mill (length 10 cm; diameter 11.5 cm; 8 steel balls 2 cm in diameter) for 5 minutes at a rotational speed of 100 rpm ¹ .

After the wear test, the mean particle diameter was still 585 μτη, corresponding to a reduction of about 22%.

Example 6 (Invention)

Example 2 was repeated by modifying that the exhaust gas from the rotary tube furnace was sucked in two places, namely in the area of introduction of amorphous sodium disilicate, and additionally at the location of the rotary tube furnace, which was approximately 2 m away from the specified area of inlet in the direction of the axis rotary tube furnaces. Both waste gas streams were combined and the solid contained therein was separated by a heat-resistant bag filter. The separated solid, together with the amorphous sodium disilicate, was reintroduced into the rotary tube furnace so that no sodium disilicate was lost. This increased the production of the rotary tube furnace to 70 kg / h, but no adhesions appeared inside the rotary tube furnace.

Example 7 (Comparative Example)

Example 2 was repeated with the change that the hot gas entering the top of the hot air spray tower had a temperature of 330 ° C. The temperature of the mixture of silicate and gas leaving the spray tower was 140 ° C. The amorphous sodium disilicate extract in the bag filter had a bulk density of 250 g / l, annealing loss at 700 ° C of 17.9 wt% and a mean particle diameter of 60μπι. This sodium disilicate is heavily dusted. Example 8 (Comparative Example)

Example 2 was repeated with the change to prepare a solution of soluble glass with a molar ratio of SiO ₂ : Na ₂ O 2.15: 1 and sprayed it in a hot air spray column into amorphous sodium disilicate with SiO ₂ : Na ₂ O 2 ratio. 15: 1. It was obtained in a rotary tube furnace at 730 ° C crystalline sodium disilicate, which showed in the X-ray diagram a line of undesirable byproduct 300 of cristobalite (SiO ₂ , which is guilty of diminishing the ability to bind lime and impairing the properties of the builder.

Example 9 (Comparative Example)

Example 2 was repeated with the change that the rotary tube furnace was insulated only so that at a temperature of 710 ° C inside the rotary tube furnace the temperature at its outer surface was at most 205 ° C. As a result, adhesions were formed on the inner wall of the rotary tube furnace on large surfaces, which often had to be mechanically repelled. A very hard, poorly crystallized product was extracted from the rotary tube furnace, which was partly the size of a soccer ball and was difficult to crush with a mechanical shredder.

Example 10 (Comparative Example)

Example 2 was repeated with the change that, by means of a mechanical shredder, the crushed sodium disilicate was ground using a mill with impact plates at 10000 min ^-1 for a mean particle diameter of 98 / im. The ground product had a gray tint and an iron content of 0.025% by weight.

Example 11 (Comparative Example)

Example 5 was repeated by changing that the compression cylinder compression pressure was only 15 kN / cm of cylinder width. The granulate obtained had a mean particle diameter of 680 / im and a bulk mass of 790 g / l. After the wear test, the mean particle diameter was only 265 / im, corresponding to a reduction of 61%. The granulate was soft and had already partially broken down into smaller agglomerates when packing.

The table shows the ability to bind lime sodium silicates with the layered structure obtained in the cases, we determined according to the following regulation:

A solution of CaCl ₂ (equivalent to 300 mg CaO) was added to 11 distilled water to give water at 30 ° dH.

To 11 of this water, which was tempered at either 20 or 60 ° C, was added 1 g in the cases of crystalline sodium silicate obtained as well as 0 to 6 ml of 1 molar glycol solution (obtained from 75.1 g glycol and 58.4 g NaCl which was dissolved in 1 L of water), after which the pH was 10.4. The suspension was stirred for 30 minutes at the selected temperature (20 and 60 ° C, respectively), keeping the pH stable. Finally, the calcium remaining in the solution was filtered off in the filtrate and complexometrically determined. From the difference against the original content we determined the ability to bind lime.

TABLE

The ability of sodium silicates to bind lime at pH 10.4 (in mg Ca / g Na ₂ Si ₂ O ₅ ) Example at 20 ° C at 60 ° C 1 74 123 2,3,4,5,10,11 82 132 6 84 136 8 75 125 9 77 126

Claims (9)

PATENT APPLICATIONS A process for the preparation of crystalline sodium silicates with a layered structure with a molar ratio of SiO ₂ to Na ₂ O (1.9 to 2.1): 1 and with a water content of less than 0.3% by weight of a soluble glass solution with at least 20% by weight of the solid, characterized in that a) obtain a solution of soluble glass by the digestion of silica sand with sodium hydroxide in a molar ratio of SiO ₂ : Na ₂ O (2.0 to 2.3): 1 at temperatures from 180 to 240 ° C and pressures from 10 to 30 bar; b) treat the solution of soluble glass in the spray drying zone in hot air from 200 to 300 ° C with a holding time of 10 to 25 s and a temperature of the waste gas leaving the spray zone 90 to 130 ° C to form a powdery amorphous sodium silicate with a content of water (determined as annealing loss at 700 ° C) of 15 to 23% by weight and a bulk density of more than 300 g / l; c) Put powdered amorphous sodium silicate containing water into an obliquely rotating tube furnace equipped with a solids-moving device and counter-flow treated with flue gas at temperatures above 500 to 850 ° C for 1 to 60 minutes at formation crystalline sodium silicate, wherein the rotary tube furnace is insulated so that the temperature of its outer wall is less than 60 ° C; d) the crystalline sodium silicate exiting the rotary tube furnace is crushed by means of a mechanical shredder to a grain size of 0.1 to 12 mm. Process according to claim 1, characterized in that the milled sodium silicate is milled by means of a mill on a grain size of 2 to 400 µm. Method according to claim 2, characterized in that a mechanical mill operating at a circumferential speed of 0.5 to 60 m / s is used. Method according to claim 2, characterized in that an air jet mill is used. Method according to claim 2, characterized in that a ball mill with a ceramic coating is used. Method according to claim 2, characterized in that a tilting mill with a ceramic coating is used. Method according to at least one of Claims 1 to 6, characterized in that the exhaust gas is aspirated from the rotary tube furnace in its middle region and in the region of its end used for introducing powdered amorphous sodium silicate and purified by means of a filter for dry dusting, quasi-continually admixing sodium silicate, which was removed from the dry dust filter, a powdery, amorphous sodium silicate containing water which is intended to be introduced into a rotary tube furnace. Method according to at least one of Claims 1 to 7, characterized in that the ground anhydrous sodium silicate is fed into a roller compacting device, which, at a cylinder pressure of 20 to 40 kN / cm, is compacted into a cylindrical width. Method according to claim 8, characterized in that the compact parts are processed after preliminary crushing by being pressed through sieves into a granulate with a bulk weight of 700 to 1000 g / l.