GB2119779A - Process for preparing alkali metal silicate solutions in a static reactor - Google Patents

Abstract

A clear-alkali metal silicate solution is obtained by passing an aqueous alkali metal hydroxide solution through a vertical tube reactor containing crystalline silica without mechanical stirring means.

Description

SPECIFICATION Process for preparing alkali metal silicate solutions in a static reactor This invention relates to a process for preparing alkali metal silicate solutions by reacting silica with an alkaline solution.

The process for preparing alkali metal silicate by alkaline fusion of silica is well known, particularly from "Soluble silicates" by J. G. Vail, Reinhold Pub. Corp. Vol. 1, p. 6 (1952). Even now it is still virtually the only process used.

Another well known process comprises autoclaving silica with an alkaline solution. Thus, "Gmelins Handbuch der anorganischen Chemie", vol. 21(1928), p. 861, cites the experiments made by Liebig (1857) and other authors, but the results yielded only silicates which were too rich in sodium hydroxide for industrial exploitation.

Patents such as British Patent No. 788 933 discuss the value of a process which is carried out at a temperature of between 1 750C and 3200C in contrast to alkaline fusion which requires temperatures around or above 1 3000C.

U.S. Patent No. 3,971,727 describes the production of alkali metal silicates in aqueous solution under pressure at a temperature of between 1380C and 2100C, but mechanical agitation and filtration are required, and the reaction involves fairly long retention times and the presence of a third product in order to arrive at worthwhile results.

European Patent Nos. 33 108 and 33 109 claim the production of sodium silicate, but the processes described require that the suspension be agitated and filtered.

The interest shown in the process of attacking silica with an alkaline solution grows with the need to save energy. The following conditions should be satisfied: the possibility of obtaining silicate solutions with a sufficiently high SiO2 content. For example, the detergent industry and the manufacture of sodium silicoaluminates (type A zeolites) require sodium silicate wherein the weight ratio of SiO2 to Na2O is similar to or higher than 2 and not more than 2.5; these solutions should be obtained continuously so as to reduce the investment costs.

We have developed a process which satisfies not only the above conditions but which also enables one to use quartz sand which has a large particle size instead of silica powders, thus saving energy on grinding, dispenses with the need to agitate the mixture of sand and solution while still ensuring a good mass transfer of reagent-sandreaction product, eliminates the need to filter the solutions obtained in apparatus especially intended for this purpose, and yields alkali metal silicates with a high weight ratio of Six, to Me2O (alkali metal oxide).

This results in a substantial simplification in the plant and at the same time great flexibility of use with a consequent considerable reduction in investment costs.

According to the present invention there is provided a process for preparing a clear alkali metal silicate solution in which the weight ratio of SiO2 to alkali metal oxide is not more than 2.5, which process comprises passing an aqueous solution of an alkali metal hydroxide through a bed of silica having an average particle size of from 0.1 to 2 mm located in a vertical tube reactor without mechanical stirring means.

The present process comprises feeding crystalline silica with an average particle size of from 0.1 mm to 2 mm and an aqueous alkali metal hydroxide solution, for example an aqueous sodium hydroxide solution, into a vertical tube which serves as the reactor. Control of the temperature and rate of introduction of the reagents and control of the concentration of the alkaline solution ensure a given concentration and weight ratio of SiO2 to Me2O in the product obtained.

The silica used is a quartz sand, the particle size of which must not be less than 0.1 mm owing to the pressure drop in the system. It must not have a particle size of more than 2 mm, so as to ensure that the reaction is not slowed down.

Although the same reaction may take place in the presence of alkaline solutions other than aqueous sodium hydroxide solutions, the latter are the ones most used on an industrial scale.

They should have an Na2O concentration of from 8.9 to 28.6% by weight at the moment of attack of the silica. In fact, too great a dilution leads to sodium silicate solutions which cannot be used directly for industrial purposes and too high a concentration produces blockages in the bed of sand owing to the formation of sodium disilicate crystals.

The reactor is fed through the top, whilst the reaction takes place on the bed of sand which serves as both the silica source and the filter, thus ensuring a good transfer of substance and the production of a clear alkali metal silicate solution.

The internal part of the reactor, which comes into contact with hot alkaline solutions, consists of a metal or alioy resistant to corrosion. Nickel is suitable for this purpose, but ordinary steels such as those used for boilers may also be used, particularly if carbonate has been added to the alkaline solution, as envisaged in French Patent No. 2462390. This reactor is fitted with a metallic grid in its lower part for holding the sand, or with any other device suitable for this purpose and having a minimum pressure drop. The cross section of the reactor determines the speed of passage of the solution through the silica. This speed should be from 2 to 1 5 m/h. Too slow a speed will not give sufficient productivity nor a correct transfer and too high a speed results in a pressure drop which makes it impossible to control the reaction.This reactor also acts as a sand filter, which involves a minimum depth of sand and yields perfectly clear solutions.

Since the weight ratio of SiO2 to Me2O and the reaction rate are dependent on the temperature in the bed of the reactor, this temperature should be from 1 50 to 2400C so as to ensure both a sufficient attack on the silica and control of the reaction. As the reaction is slightly exothermic, there is no need to heat the reactor. However, the alkaline solution should be introduced at a temperature such that the temperature in the reactor is maintained. The temperature of the products used is sufficient to ensure concentration of the solution, and/or preheating of the reagents.

In a preferred embodiment of the process according to the invention, the aqueous alkali metal hydroxide solution entering the reactor is formed so that there is no need for any external application of heat energy.

The particularly advantageous result obtained with such a method is achieved by virtue of the judicious use, on the one hand, of the calories available in the alkali metal silicate solution leaving the reactor to preheat a concentrated aqueous solution of an alkali metal hydroxide and water and, on the other hand, of the calories supplied by the exothermic dilution of this concentrated alkaline solution with water to form the alkaline solution entering the reactor.

Thus, when an aqueous sodium hydroxide solution is used, it is possible, for example, to obtained clear sodium silicate solutions containing from 35% to 46% by weight of sodium silicate, without any external application of heat energy.

The particularly advantageous result of the process according to the invention, in its preferred embodiment, is not only the saving in heat energy achieved compared with a different method of operation within the scope of the invention, but also the possibility of obtaining, economically, a clear alkali metal silicate solution which can be used directly in industry.

The present invention will now be described, by way of example only, with reference to the accompanying drawing which shows a diagram of a process according to the preferred embodiment of the invention: The silica, e.g. a quartz sand, extracted from storage containers, for example, by means of a known system allowing continuous passage, at a pressure above atmospheric pressure, of solid products which are initially at atmospheric pressure, is fed through a duct 1 into the upper part of a static reactor 2. The bed of sand which forms in the lower part of this reactor is supported by any device, such as a grid or wire gauze, (not shown in the drawing) which is suitable for this purpose and which gives a minimal pressure drop.

A concentrated aqueous solution of an alkali metal hydroxide, supplied through a pipe 3, is preheated at 4 by indirect heat exchange with the alkali silicate solution circulating in a duct 5. The solution is then homogeneously diluted at 6 with water fed in through a pipe 7, which is itself preheated at 8 by indirect heat exchange with the alkali metal silicate solution circulating in a duct 9. The alkaline solution thus obtained enters the upper part of the reactor 2, above the bed of silica, through a pipe 10.

The alkali metal silicate solution which continuously flows out of the bottom of the reactor 2 through a pipe 11 is first conveyed through ducts 5 and 9 and used to preheat the concentrated alkaline solution and the water in pipes 3 and 7 and is then recombined in a duct 12 and collected in suitable receptacles.

The following Examples illustrate the process according to the invention. Examples 9 and 10 illustrate a preferred embodiment of the invention.

Example 1 A charge of 53 kg of quartz sand with an average particle size of 300 m is introduced into an insulated vertical nickel tube 6 m long and with an internal diameter of 90 mm. An aqueous sodium hydroxide solution containing 19.6% by weight of Na2O is passed through for 1 h 20 min at a flow rate of 50 I/h. The temperature in the tube is maintained at 2250C.

At the bottom of the reactor, a sodium silicate solution is collected, containing 1 94 g/l of Na2O and 485 g/l of SiO2.

The unreacted sand remains in the reactor, which is recharged with fresh sand for the next reaction.

Example 2 An insulated vertical nickel tube with an internal diameter of 90 mm and a length of 2 m is continuously fed with quartz sand with an average particle size of 300 ym at a rate of 22 kg/h and a hot aqueous sodium hydroxide solution containing 11.2% by weight of Na2O at a rate of 69 I/h of solution. The temperature in the tube is maintained at 2200 C.

The silicate solution formed at the bottom of the reactor contains 125 g/l of Na2O and 306 g/l of SiO2.

Example 3 Silica in a quantity of 27 kg/h and an aqueous sodium hydroxide solution containing 17.6% by weight of Na2O at a rate of 50 I/h of solution are continuously fed into the tube used in Example 2.

The silica is a quartz sand with an average particle size of 850 m. The temperature in the reactor is maintained at 21 80C. The silicate solution formed at the bottom of the reactor contains 1 75 g/l of Na2O and 429 g/l of SiO2.

Example 4 Silica in a quantity of 8.4 kg/h and an aqueous sodium hydroxide solution containing 19.6% by weight of Na2O at a rate of 33 Ijh of solution are continuously fed into the tube of Example 2. The silica is a quartz sand with an average particle size of 300,us. The temperature in the reactor is maintained at 1600C. The silicate solution formed at the bottom of the reactor contains 1 97 g/l of Na2O and 200 g/i of SiO2.

Example 5 Silica in a quantity of 25 kg/h and an aqueous sodium hydroxide solution containing 23.4% by weight of Na2O at a rate of 40 I/h of solution are fed continuously into the tube of Example 2. The silica is a quartz sand with an average particle size of 300 ym. The temperature in the reactor is 1 900 C. The silicate solution formed at the bottom of the reactor contains 226 g/l of Na2O and 454 g/l of SiO2.

Example 6 A vertical mild steel tube fitted with a double jacket, having an internal diameter of 90 mm and a length of 2 m, is continuously fed with silica at a rate of 23 kg/h and a solution containing 14.9% by weight of Na2O and 20 g of Na2CO3 at a rate of 50 I/h of solution. The silica is a quartz sand with an average particle size of 300 ,um. The temperature in the tube is maintained at 21 80C.

The pressure in the reactor is maintained by regulation. The silicate solution formed at the bottom of the reactor contains 160 g/l of Na2O and 384 g/l of SiO2.

Example 7 The tube described in Example 2 is continuously fed with silica at a rate of 21 kg/h and aqueous sodium hydroxide solution containing 16.7% by weight of Na2O at a rate of 50 I/h of solution. The silica is a quartz sand with an average particle size of 130 ,um. The temperature in the reactor is maintained at 1 900C. The silicate solution formed contains 165 g/l of Na2O and 338 g/l of SiO2.

Example 8 The tube described in Example 2 is continuously fed with silica at a rate of 31 kg/h and aqueous potassium-hydroxide solution containing 26.7% by weight of K2O at a rate of 50 I/h of solution. The silica is a quartz sand with a particle size of 300 ssum on average. The temperature in the reactor is maintained at 1 950C. The silicate solution formed at the bottom of the reactor contains 264 g/l of K2O and 462 g/l of SiO2.

Example 9 1 3.3 kg/h of quartz sand with an average particle size of 300,um and at a temperature equal to ambient temperature is introduced into the upper part of a reactor consisting of a vertical cylindrical tube of ordinary steel, carefully insulated like the whole apparatus, with an internal diameter of 90 mm and 6 m in height.

33.6 kg/h of a concentrated aqueous sodium hydroxide solution containing 19.8% by weight of Na2O and at a temperature of 19200 are introduced ito the upper part of the same reactor, above the bed of sand.

This alkaline solution at 192 is obtained by homogeneously and exothermically mixing 1 7.9 kg of a concentrated aqueous sodium hydroxide solution containing 37.2% by weight of Na2O and heated to 1 71 0C by indirect heat exchange with the sodium silicate solution leaving the reactor at 188 C, with 15.7 kg/h of water heated to 171 OC by heat exchange as defined above.

The clear sodium silicate solution, produced at the rate of 46.9 kg/h, contains 42.6% by weight of sodium silicate wherein the weight ratio of SIO2 to Na2O is equal to 2.

Example 10 In the same apparatus, using the same procedure and the same quantity of sand as in Example 9, sodium silicate is prepared by introducing into the reactor 25.5 kg/h of sand at ambient temperature and 59.5 kg/h of an alkaline solution containing 21.4% by weight of Na2O at a temperature of 21 30C.

This alkaline solution at a temperature 2130C is obtained by homogeneously and exothermically mixing 34.27 kg/h of a concentrated aqueous sodium hydroxide solution containing 37.2% by weight of Na2O and heated to a temperature of 19200 by indirect heat exchange with the sodium silicate solution leaving the reactor at a temperature of 2070C, with 25.23 kg/h of water brought to a temperature of 1920C by heat exchange as described above.

The clear sodium silicate solution, produced at a rate of 85 kg/h, contains 45% by weight of sodium silicate wherein the weight ratio of SiO2 to Na2O is equal to 2.

Claims (12)

Claims

1. Process for preparing a clear alkali metal silicate solution in which the weight ratio of SiO2 to alkali metal oxide is not more than 2.5, which process comprises passing an aqueous solution of an alkali metal hydroxide through a bed of silica having an average particle size of from 0.1 to 2 mm located in a vertical tube reactor without mechanical stirring means.

2. Process as claimed in Claim 1, wherein the alkali metal hydroxide solution passes through the bead of silica at a speed of from 2 to 1 5 m/h.

3. Process gas claimed in Claim 1 or 2, wherein the temperature at which the silica is attacked by the aqueous alkali metal hydroxide solution is from 1500C to 24000.

4. Process as claimed in any one of Claims 1 to 3, wherein the aqueous solution of an alkali metal hydroxide entering the reactor is an aqueous sodium hydroxide solution containing from 8.9% to 28.6% by weight of Na2O.

5. Process as claimed in any one of othe preceding claims, wherein the aqueous alkali metal hydroxide solution introduced into the reactor is formed by homogeneously and exothermically mixing a concentrated aqueous alkali metal hydroxide solution and water, each preheated by indirect heat exchange with the alkali metal silicate solution leaving the reactor.

6. Process as claimed in any one of the preceding claims, wherein the silicate product is a sodium silicate solution having a weight ratio of SiO2 to Na2O equal to 2.

7. Process as claimed in any one of the preceding claims, wherein the silica used is a quartz sand.

8. Process as claimed in any one of the preceding claims, wherein the bed of silica acts as a filter for the product obtained.

9. Process as claimed in any one of the preceding claims, wherein the alkali metal hydroxide solution contains an alkali metal carbonate.

1 0. Process according to Claim 1 substantially as hereinbefore described with reference to the accompanying drawing.

11. Process according to Claim 1 substantially as described in any one of the foregoing Examples.

12. An alkali metal silicate whenever prepared by a process as claimed in any one of the preceding claims.