MXPA97002334A - Clear aqueous soil silicate solutions - Google Patents

Abstract

A method for making a clear aqueous solution of sodium silicate from a mixture of silicon dioxide, sodium carbonate, and water is described. A fusion of the mixture of silicon dioxide and sodium carbonate is formed, the melt is cooled to form solid sodium silicate, the solid sodium silicate is mixed with water and an inhibiting agent to form a mixture containing from about 10 to about 2000 ppm of metallic impurities, and the mixture is heated under pressure to dissolve this sodium silicate in the water.

Description

CLEAR AQUEOUS SOLUTIONS OF SODIUM SILICATE BACKGROUND OF THE INVENTION This invention relates to a method for making a clear aqueous solution of sodium silicate from dioxide of silicon, sodium carbonate and water containing about 10 to about 2000 ppm total of metal impurities. In particular, it relates to forming a fusion of a mixture of silicon dioxide and silicon silicate, cooling the melt to form solid sodium silicate, mixing the solid sodium silicate with water and an inhibiting agent to form a mixture, and Heat the mixture under pressure to dissolve the sodium silicate in the water. Aqueous solutions of sodium silicate are made by melting sand and soda ash in an open oven. The melt is diverted out and poured into molds to form 1 bricks. The bricks are dissolved in hot water under pressure to form a aqueous solution of sodium silicate. The solution is used to make adhesives and feed materials to make catalysts and detergents, in paper bleaching, and in many other industrial applications. Aqueous solutions of sodium silicate can be clear or nebulous. Nebulous solutions are not acceptable to many customers, mainly due to aesthetic reasons. The cloudiness is due to the precipitation of silicates and impurities. Unsatisfactory efforts have been made to prevent the formation of haze by altering the pressure and temperature at which the sodium silicate is dissolved. Despite many efforts to solve this problem, a publicly known method is not available, good for preventing silicate from precipitating at commercially acceptable concentrations of solution. The sodium silicate solution, of course, can be clarified by removing the precipitate, but that is usually too expensive to be practical.

SUMMARY OF THE INVENTION We have discovered that the presence of metallic impurities in aqueous solutions of sodium silicate is responsible for the precipitation of the silicates. These impurities, which probably come from silicon dioxide, sodium carbonate, or water, are present in very low concentrations, apparently as seeds on which the sili cates can precipitate. It has also been discovered that the precipitation of silicones can be prevented easily, even when the metallic impurities are present in significant concentrations, adding a chelating or inhibiting agent to the solution before it becomes cloudy. If this is done, the solution remains clear.

Brief Description of the Drawings Figure 1 is a graph of product turbidity see its EDTA concentrations, as explained in Example 5. Figure 2 is a graph showing the effects of various inhibition agents on turbidity, as explained in Example 6.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In the process of this invention, an aqueous solution of sodium silicate is prepared in a series of steps. In the first step, silicon dioxide and sodium carbonate are mixed together. Silicon dioxide is commonly used in the form of sand and sodium carbonate is sold as soda ash This invention is applicable only to silicon dioxide and sodium carbonate mixtures containing from about 10 to about 2000 ppm, calculated as calcium, of metallic impurities. If the total concentration of metal impurities is greater than approximately 2000 ppm, its concentration in the raw materials should be reduced to less than 2000 ppm by other less expensive methods, suitable for the raw material before treatment in accordance with the method of this invention. tion. If the concentration of metallic impurities is less than about 10 ppm, impurities are unlikely to precipitate sufficiently from the silicates to cause haze. Examples of metal impurities that may precipitate include calcium, magnesium and iron, and possibly aluminum, titanium, manganese and zinc. The silicon dioxide and sodium carbonate are mixed - stoichiometrically to achieve the desired ratio of Si02 to Na20 in the sodium silicate product. The ratio of Si02 to Na20 is typically between about 2 and about 3.5, but the invention is more applicable at higher ratios of about 2.5 to about 3.5 since there is a greater tendency for the solution to nebulize at those ratios. In the next step, the mixture of silicon dioxide and sodium carbonate is melted. The melt is conducted sufficiently above the melting point of the sodium silicate below the viscosity of the melt sufficient to flow easily; Approximately 1350QC is typical. The molten sodium silicate glass bed solidifies, typically in the briquette form, although other forms may also be used. In the next step, water is added to the solidified sodium silicate, which has been cooled to approximately 800 ° C or less. The inhibiting agent is preferably added after the water and must be added before the next step so that it is effective to prevent the formation of haze. The addition of the inhibiting agent after the solution has been made -nebulous will not clarify the solution. The inhibiting agent is a compound that binds to the metallic impurity and keeps it in solution. The inhibiting agent must be stable under the conditions required to dissolve the sodium silicate. The peptide axes of inhibiting agents that can be used include ethanediamine tetraacetic acid, especially its disodium salt (referred to herein as EDTA), nitrilotriacetic acid (NTA) sodium hexametaphosphate (HMP), sodium oxalate (SO), and sodium citrate (SC). The preferred inhibitory agent is EDTA because it was the most effective inhibiting agent tested. The amount of inhibition agent used must be at least stoichiometric with the amount of impurities that are present. For the purpose of this invention, only calcium, magnesium and iron are considered when calculating the stoichiometry. (The concentration of impurities can be determined by elemental analysis). EDTA can be used in stoichiometric amounts, but it may be necessary to use an excess of an effective inhibiting agent. The amount of water that can be added to the solid sodium silica will vary with the needs of the customer, but a mixture of about 10 to about 70% by weight of water and about 30 to about 90% by weight of silicon Sodium is typical. In the next step of the process of this invention, the mixture of the solid sodium silicate, water, and inhibiting agent is heated under pressure to dissolve the sodium silicate in the water until the desired concentration is reached. If the concentration is too high, the solution will be too viscous to be easily handled, and simple concentrations of sodium silicate are not economical; a solution concentration of about 60 to 70% by weight of water is preferred.

A pressure from about atmospheric to about 1 MPa - (10.55 kg / cm) can be used, but the preferred scale is from -about 0.4 to about 0.8 MPa (about 4.22 to -about 8.44 kg / cm). The temperature may be from about 60 to about 185dC and is preferably from about 150 to about 1809C. The sodium silicate solution is then cooled, or allowed to cool, to room temperature. The following examples further illustrate this invention. In the examples, the silicate turbidity was measured in Nephelometric Turbidity Units (NTU) using a Trubidím tro Model 18900-00 Hach ^ Ratio. The Nefelo metric angle measurement (909) is expressed as the ratio of the scattered light 90 to the sum of transmitted light and scattered light measurements to the front. The levels of impurity were quantified by elemental analysis of raw materials and sodium silicate solution using atomic emission spectrometry of inferentially coupled plasma (ICP-AES). The impurities considered were Ca, Mg, and Fe. For comparison purposes, all impurity concentrations are calculated as Ca on a weight basis of dissolver water. In Examples 1 to 6, the laboratory dissolver used a Parr pump coated with a polytetrafluoro polymer and used 23 grs. of sodium silicate glass and 30 grs. of water as the reaction mixture EXAMPLE 1 Raw materials of low level of impurity: Low impurities = Low liquidity. Unim Iota 6 sand and soda ash D) Chempure were mixed in the appropriate proportions and melted at 1350 C (for 15 to 90 minutes) to produce a silicate glass of 3.22 ratio. The glass was dissolved in an appropriate amount of deionized water (DI) (at 135 ° C) in a laboratory dissolver to produce a solution of 40 μm or greater. The impurity levels were approximately 21 ug / g as Ca on a weight basis of dissolver water. The turbidity of the resulting product was 10 NTU.

EXAMPLE 2 Raw Materials of Plant: Medium level impurities = moderate turbidity Sand and soda ash obtained from production isntalations were mixed in the appropriate proportion and melted at 1350 ° C (for 15 to 90 minutes) to produce a calcium silicate glass. ratio of 3.22. The glass was dissolved in an appropriate amount of water (at 135 ° C) in a laboratory dissolver to produce a solution of 40 baumé or higher. The impreza levels were approximately 140 ug / g as Ca on a weight of dissolver water. The resulting product was 20 NTU.

EXAMPLE 3 Plant silicate glass: Medium level impurities = moderate turbidity A sodium silicate glass of 3.22 ratio obtained from a production isenta- lation was dissolved in an appropriate amount of DI water (at 135 ° C) in a dissolver. of the laboratory to produce a solution of 40 baumé or greater. The impurity levels were approximately 170 ug / g as Ca on a weight basis of dissolver water. The turbidity of the resulting product was 24 NTU.

EXAMPLE 4 Plant silicate glass with impurity added to water: High level impurities = High turbidity A glass of sodium silicate with a ratio of 3.22 obtained from a production isentation was dissolved in an appropriate amount of DI water at 1359 C (forced with Ca) in a laboratory dissolver to produce a solution of 40 baumé or greater. The impurity levels were approximately 250 ug / g as Ca on a weight basis of dissolver water. The resulting product turbidity was 150 NTU.

EXAMPLE 5 Silicate glass of plant with impurity + EDTA added to water: High Impurities + EDTA = Low Turbidity A 3.22 ratio sodium silicate glass obtained from a production facility was dissolved in an appropriate amount of DI water at 135 QC (forced with Ca and EDTA) in a laboratory solvent to produce a solution of 40 baumé or higher. The impurity levels were approximately 250 ug / g as Ca on a weight basis of dissolver water. The EDTA level was 0.005 molar on a dissolver water base. The turbidity of the resulting product was 9 NTU. Figure 1 graphically represents the amount of EDT required to inhibit all impurities when the original level of impurities is 250 ug / g as Ca. In Figure 1, the order is NTU and the abscissa is the molar concentration of EDTA. EDTA inhibits the impurities in a sequential manner, acting on the imprezas in the dissolver water first and those in the second silicate glass. Area A represents the impurities inhibited by EDTA that originated in the water (this is approximately 0.0015 M). Area B represents the impurities inhibited by EDTA that originated in the silicate glass is approximately 0.004 M) and area C represents the region where the impurities have been inhibited and no further turbidity improvement is observed. of product with an increase in EDTA concentration. As shown, the calculated implicit molarities correlate well with the "breakpoints" in the one presented in Figure 1.

EXAMPLE 6 Silicate glass from plant with impurity and other in? bition added to water. A sodium silicate glass of 3.22 ratio obtained from a production facility was dissolved in an appropriate amount of DI water at 1359C) (forced with Ca and inhibiting agent) in a laboratory dissolver to produce a 40% solution. Baumé or greater. The impurity levels were approximately 250 ug / g as Ca on a weight basis of solvent water. The turbidity of the resulting product is illustrated graphically in Figure 2, where the ordinate is NTU and the abscissa is the molar concentration of inhibiting agent. As shown in Figure 2, different inhibiting agents have different abilities to inhibit turbid impurities, EDTA being the preferred inhibiting agent.

EXAMPLE 7 1000 g of sodium silicate glass d 3.22 ratio was dissolved in 1689 g of deionized water (DI) at 165 ° C. for 4.5 hours in a 4.5 liter stainless steel autoclave, to produce a solution of 40 ° or higher. Two forced dissolution tests were performed. The control used water of dissolution and 75 ppm of Ca without EDTA; the second test used dissolution water containing 75 ppm of Ca and 0.02 M of EDTA. A total of three dissolution tests were performed: (1) baseline using only DI water, (2) DI water - enforced with Ca, and (3) forced DI water with both Ca and EDTA. The results are presented in the following table: Test Turbidity (NTU Baumé (9Be) Baseline 31 34.8 Forced with Ca 81 34.2 Forced with Ca and EDTA 18 34.8 As indicated by the data, there is an improvement in product turbulence with the addition of EDTA. This improvement (as shown in previous examples) goes beyond that achieved by DI water alone. This test indicates that the higher temperature and pressure of the autoclave (compared to the PARR Pump method) do not adversely affect the ability of the eDTA to clarify sodium silicate solutions. During the course of this experiment, it was observed that a large amount of white precipitate was adhering to the side of the autoclave after the completion of the Ca fox test. This white precipitate was not evident after the dissolution test containing both Ca and eDTA. This observation may suggest that Ca causes a white film to "build up" on the walls of the dissolver and that EDTA can prevent this type of film from accumulating.

Claims (20)

CLAIMS:

1. - A method for making a clear aqueous solution of sodium silicate from silicon dioxide, sodium carbonate and water, comprising: (A) forming a mixture of silicon dioxide and sodium carbonate; (B) melting the mixture to form fumed sodium silicate; (C) cooling the molten sodium silicate to form solid sodium silicate; (D) adding water and inhibiting agent to the solid sodium silicate to form a mixture containing about 10 to about 2000 ppm of metallic impurities; and (E) heating the mixture under pressure to dissolve the sodium silicate in the water.

2. A method according to claim 1, wherein the silicon dioxide is sand and the sodium carbonate is soda ash.

3. A method according to claim 1, wherein the silicon dioxide and the sodium carbonate are in a weight ratio calculated as S iO to Na20 of 2.5 to 3.5.

4. A method according to claim 1, wherein the solid sodium silicate is in the form of briquettes.

5. A method according to claim 1, wherein the inhibiting agent is selected from the group consisting of ethylenediamine tetraacetic acid, nitrile triacetic acid, sodium hexametaphosphate, sodium oxalate and sodium citrate.

6. A method according to claim 1, wherein the mixture is heated to 60 to 1859C at an atmospheric pressure to 1 MPa.

7. A method according to claim 1, wherein the mixture is from about 10 to about 70% by weight of water and from about 30 to 90% by weight of silicate of dioxide.

8. An aqueous solution of sodium silicate made in accordance with the method of claim 1.

9. In a method for making an aqueous solution of sodium silicate wherein a mixture of silicon dioxide and sodium carbonate. it is heated to form a sodium silicate melt, the melt is solidified, water is added to the solid sodium silicate, and the water and the solid sodium silicate are heated under pressure to dissolve at least part of the solid sodium silicate in the water forming a composition containing a total of about 10 to about 2000 ppm of metallic impurities the improvement comprising adding an inhibiting agent to the water before the water and the solid sodium silicate are heated.

10. - A method according to claim 9, wherein the silicon dioxide is sand and the sodium carbonate is soda ash.

11. A method according to claim 9, wherein the silicon dioxide and the soda ash are in a weight ratio calculated as Si02 to Na20 of 2.5 to 3.5.

12. A method according to claim 9, wherein the solid sodium silicate is in the briquette form.

13. A method according to claim 9 wherein the inhibiting agent is selected from the group consisting of eti lendiamine tetraacetic acid, nitrile triacetic acid, sodium hexametaphosphate, sodium oxalate and sodium citrate.

14. A method according to claim 9 wherein the mixture is heated to 60 to 1859C at an atmospheric pressure to 1 MPa.

15. A method according to claim 9 wherein the water is from about 10 to about 70% by weight of the total weight of water plus sodium silicate.

16. An aqueous solution of sodium silicate made in accordance with the method of claim 9

17. A method for making a clear aqueous solution of sodium silicate, comprising: (A) forming a first mixture of sand and ash of so sa in a weight ratio calculated as Si02 to Na20, from 2.5 to 3.5; (B) heating the first mixture to a temperature at least sufficient to form a melt of no sodium cato; (C) pouring the melt into briquette molds and cooling the melt to form solid sodium silicate briquettes; (D) form a second mixture of the briquettes with Water, wherein the second mixture contains from about 10 to about 2000 ppm of metallic impurities; (E) adding to the second mixture an amount of an inhibition period selected from the group 15 which consists of eti lendiamine tetraacetic acid, nitrilotriacetic acid, sodium hexametaphosphate, sodium oxalate, and sodium citrate at least stoic acid with the amount of metal impurities; (F) heating the second mixture under pressure to form an aqueous solution of about 30 to about 40% by weight of sodium silicate; and (G) cooling the aqueous solution to room temperature

18. A method according to claim 1 Wherein the inhibiting agent is the disodium salt of eti len-diamine tetraacetic acid.

19. A method according to claim 1 wherein the second mixture is heated to 60 to 1859C at an atmospheric pressure at 1 MPa.

20. An aqueous solution of sodium silicate made in accordance with the method of claim 17.