CA1220306A - Two-stage process for the production of concentrated sodium silicate solutions from sodium silicate lump glass and thin liquors - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE

Aqueous sodium silicate solutions containing at least 26% of SiO2 and having a ratio of weight of SiO2 to Na2O of from 2.0 to 3.0 can be obtained using thin or waste liquor containing at most 20% of NaOH
providing: A) solid sodium silicate glass having a ratio by weight of SiO2 to Na2O of from 3.0 to 3.5 is first dissolved in alkali-free water at 135° to 155°C
under the autogenous vapor pressure of the water until an aqueous sodium silicate solution containing more than 20% by weight and less than 30% by weight of SiO2 is formed, after which thin liquor containing at most 20%
of NaCH is added in such a quantity as to produce ratios by weight of SiO2 to Na2O of from 1.8 to 2.6 in the mixture of the two components; B) more solid sodium silicate glass is dissolved in this mixture until a concentration of at least 23% of SiO2 in solution is reached. A high volume-time yield for minimal energy consumption and maximal utilization of dilute waste liquor is obtained.

Description

B~CKGROU~JD OF TAO I~~JENTION
This invention relates to the production of aqueous concentrated sodium silicate solutions using so-called thin liquors by a -two-stage process in winch sodium silicate lass is first redissolved in alkali-free water to an SiO2-content in solution of at least 20% by weight and, after dilution with thin liquor, is further dissolved with addition of more sodium silicate glass until the required Sue concentration of at least 23%, preferably at least 26%, by weight is reached.
On an industrial scale, sodium silicate solutions, hereinafter referred to as -wa-terglass-~
solutions, are preferably produced by dissolving solid sodium silicate glass, hereinafter referred to as fusible glass , in water at 135 to 155~C under the corresponding autogenous pressure and, after production, preferably have high Sue concentrations of from 26% to 30~ by weight.
On an industrial scale, the solid glass for these aqueous solutions is melted, in general using soda as the alkali component and with ratios by weight of Sue to NATO thereinafter referred to as ratio of from 3.0 to 3.5 and, more particularly, from 3.3 to I
To produce aqueous solutions having lower Sue to NATO ratios, for example of down to 2.0, it has hitherto been standard practice either to melt and then dissolve special and expensive alkaline fusible glasses with correspondingly lower ratios of down to >2.0 for the dissolution process or, alternatively, to dissolve readily available solid lo - 2 Nab/

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, ` glasses characterized by ratios of from 3.3 to 3.4 in the usual way, to adjust the water glasses with concentrated sod-I'm hydroxide to the required, resulting ratio and then to concentrate this adjusted aqueous solution by evaporation to the required SiO2-concentrations. The use of waste or thin liquors, that is, liquors of low sodium hydroxide content, increases the concentration costs in direct dependence upon the alkali content of the liquor used and, in addition, sign-l ` i~icantly reduces the volwme/tim~ yield of the dissolving/con-cent rating process.
Hitherto, it has not been possible to directly dissolve fusible glass in "thin liquor", i.e. in sodium hydroxide solution having an Noah content of from 5 Jo 20% by weight (preferably from 10 to 14~ by weight), rather than in alkali-free water. Depending upon the initial NaOH-content oath to "dissolving water", the addition of/liquor leads to a signify cant increase in the dissolution time, but without the nieces-spry concentrations of from about 15% to 20% of Sue being obtained in acceptable reaction times (twice to three times the otherwise usual dissolving time without the addition of us alkali). Also, the addition of/liquor causes the solid glass to clump-up, which interferes seriously with the dissolving process.

OBJECTS OF THE INVENTION
An object of the present invention is to provide a pro- i cuss for the production of concentrated alkaline sodium silicate solutions using the alkali content of dilute thin or waste liquors, in which the disadvantages mentioned above are . ,. . . . , . .. i . . . . .

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avoided.
A further object of the present invention is to obtain as high a volume/time yield as possible for as low an energy consumption as possible in the dissolving processes while, at the same time, ensuring maximal utilization of dilute waste liquors.
Another object of the present invention is the development of a process for the production of aqueous sodium silicate solutions containing at least 23% by weight of Sue and having a ratio by weight of Sue to NATO of from 2.0 to 3.0 using thin liquor containing at most 20% by weight of Noah and solid sodium silicate glass having a ratio by weight of Sue to NATO
of from 3.0 to 3.5, consisting essentially of the steps of a) redissolving said solid sodium silicate glass in alkali-free water a-t 135 to 155C and under the corresponding autogenous vapor pressure of the water until an aqueous sodium silicate solution containing.
at least 20~ to at most 30% by weight of Sue is obtained, b) adding said thin liquor to said aqueous sodium silicate solution in such a quantity as to produce , ratios by weight of Sue to NATO of from 1.8 to

2.6 in the mixture of the two components with a solid content dependent upon the concentration of 2$ the thin liquor, c) dissolving more solid sodium silicate glass in this `.
mixture in a second dissolving step at 135 to 155C~

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,, , and under the corresponding autogenous vapor pressure us-ill a concentration of at least 23~ of Sue in solution is reached, and d) recovering said aqueous sodium silicate solution.
These and other objects of the invention will become more apparent as the description thereof proceeds.
'.
I DESCRIPTION OF THE INVENTION
.. . . . _ _ It has now surprisingly been found that, despite the 'addition of alkali in -the form of thin liquor in a quantity of imp to 15 kg of NATO per 100 kg of Sue in the final solution, to a solution where the fusible glass has been dissolved be-forehand with alkali free water up to a concentration of at least 20 to at most 30~ by weight of Sue in solution, fusible glass continues to dissolve therein without any significant reduction in the rate of dissolution.
Accordingly, the present invention relates to a process for the production of aqueous sodium silicate solutions con-twining at least 23% by weigh-t of Sue and having a ratio by weight of Sue to NATO of from 2.0 to 3.0 and preferably from 2.7 to I using thin liquor containing at most 20% by weight and preferably less than 14% by weight Naomi and solid sodium silicate glass having a ratio by weight of Sue to NATO of from 3.0 to 3.5 and preferably from 3.0 to 3.4, characterized in that, a the solid glass is first redissolved in alkali free water at 135 -to 155C and under the corresponding autogenous vapor pressure of the water until an aqueous sodium silicate solution containing at least 20~ by weight and at most 30% by weight of Sue is I obtained, , I
b) the thin liquor is then added in such a quantity as to produce ratios by weight of Sue to Noah of from 1.8 to 2.6 in the mixture of -the -two components and a solids content dependent upon the concentra-lion of the thin liquor, c) after which more solid sodium silicate glass is dissolved in this mixture in a second dissolving step until a concentration of at least 23~ of So in solution is reached.
More particularity the present invention involves a pro cuss for the production of aqueous sodium silicate solutions containing at least 23% by weight of Sue and having a ratio ;
by weight of Sue to Noah of from 2.0 to 3.0 using thin liquor containing at most 20~ by weight of Noah and solid sodium silicate glass having a ratio by weigh-t of Sue to Noah of from 3.0 to 3.5, consisting essentially of the steps of a) redissolving said solid sodium silicate glass in alkali-free water at 135 to 155C and under the corresponding autogenous vapor pressure of Ed I
the water until an aqueous sir silicate solution containing at least 20% to a most 30% by weight of Sue is obtained, b) adding said thin liquor to said aqueous sodium silicate solution in such a quantity as to produce ratios by weight of Sue to Nephew from 1.8 to 2.6 in the mixture ox the two components with a solid content dependent upon the concentration of the thin liquor, c) dissolving more solid sodium silicate glass in this mixture in a second dissolving step at 135 to 155C

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c) keynoted.
dud under the corresponding autogenous vapor pressure until a concentration of at least 23~ of Sue in sol-union is reached, and d) recovering said aqueous sodium silicate solution.
The sodium silicate solution obtained as described above may then be separated off with recovery of heat and optionally filtered, followed by the addition of more thin liquor The process according to the invention may be carried out in conventional equipped dissolving reactors. The dissolving reactors may be-formed, for example, my an axially suspended vessel designed to rotate about a longitudinal axis or even by a static vessel in which provision is made for adequate intermixing of the vessel contents by pump-recirculation of the liquid phase through solid fusible glass lumps resting on sieve plates. In order to ensure as low an energy consumption as possible in the dissolving process, dissolving vessels of the type in question are conventionally equipped with heat :
recovery systems.
In the first stage of the process according to the invent lion, the dissolving vessel is first filled with preferably hot lump glass in a large excess of up to 200%, based on the resulting water glass solution. Thereafter, a little cold water is added to force open the hot pieces of fusible glass lumps, followed immediately afterwards by the addition under normal Pressure and at a temperature of up to lQ0C of most of the dissolving water heated via the heat recovery system.
The reactor is then closed. In order to minimize outlay on ; .
hardware and to obtain high volume/time yields, the dissolving process is carried out at 135 to 155C and preferably at 1~0 ,~.... ii ,, , !

~ZZ(:J3(16 to 150C under the corresponding autogenous vapor pressure owe the water. In cases where hot fusible glass lumps are used as the starting material and where mainly hot dissolving water is used, only a little additional heating energy, for example steam under 4 bars, need be added to the reaction mix lure, by virtue of the exothermic nature of the dissolving process, to adjust the preferred dissolving temperature of 140 to 150C. After only about 60 to 90 minutes' dissolutiv at that temperature, the aqueous solution has Sue contents of as high as 20% to 30% and, commensurate with the ratios prevailing in the solid glass used, associated alkali content.
In conventional dissolving processes the solution is expanded to normal pressure, i. e. cooled to around 100C, by' transfer under its own pressure to a suitable expansion apparatus, the water vapor formed during expansion it subset connately employed to heat dissolving water for a following disk solving process up to a temperature of 100C. Depending on the application envisaged, the cooled water glass is then I, Qr~,J/~~ e optionally adjusted -to the required ratio of Sue to NATO\
and/or concentrated by evaporation to a higher solids content In the process according to the present invention, how-ever, the thin liquor, after the desired Sue con-tent in Swahili lion has been reached, is pumped into the reactor and hence directly into the 135 to 155C hut water glass solution and `
the reaction continued in -the second state of the dissolving process according to the invention until a sufficiently con- -cent rated water glass solution is again obtained. The water-glass solution thus obtained may then be subjected in the , .

, usual way -to further treatments, such as expansion, filter-lion, adjustment or concentration by evaporation, depending on the purpose for which it is to be used.
Depending on the apparatus used, however, it may be tech Nikolai advisable to carry out preparation of the water glass solution on the one hand and dissolution on the other hand in' ; different dissolving reactors, that is, to separate the two process steps in terms of space, especially if different mat-. trials are optimal for both dissolving processes, depending ' , on the mixing and concentration conditions. Accordingly, the' ' water glass solution to be initially introduced may be pro- -pared for exempt' in the usual way in static dissolvers and, . after expansion to normal pressure with recovery of energy, transferred to another, for example rotating dissolving reactor in which thin liquor is introduced under normal Ares-' sure, thereby saving pressure pumps, followed by treatment in' the rotating dissolver with addition of hot solid glass/
again at 135 to 155C and preferably at 140 to 150C, until' the required Sue to NATO ratio is reached. This procedure is that employed in most of the following examples.

EXAMPLES
The invention is illustrated by, without being limited to, the following Examples.
Jo The test on which the Examples are based were carried 25 ` out on an industrial scale using starting materials of uniform quality. The fusible glass used was a technical sodium Swahili gate lump glass having the composition NATO . 3.35 Sue with . . .

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it total impurity content of less than I by weight. Other than in Example 8, the water glass solution used was an aqueous solution of this fusible glass containing 8.0% of NATO and 26.8~ of Sue. This water glass solution was prepared in a separate step as indicated above, employing demineralized water and fusible glass. The temperature was maintained at 145 -t 2C during dissolutiorl. The waste liquor used in the Examples contained 14.0% by weight of Noah. Lower concentra lions of sodium hydroxide in the thin liquor were adjusted in each case by the addition of demineralized water.
The components were introduced hot (solid glass at around 200C, thin liquor and water glass solution at around 70 to 95C).
In all the Examples, a temperature of 145 2C was established by the direct introduction of steam into the react lion mixture. After a reaction time of 120 minutes in each case, samples of the resulting solutions were taken and anal-Swede for their NATO and Sue corltents.
The quantities of individual components used and also the compositions of the mixtures are shown in Table 1. Table 2 shows the yields and compositions of the solutions obtained after hydrothermal treatment.
Examples 2 to 5 (1 = Comparison Example) were carried out in a reactor lined with steel plates which had a capacity of 24 cubic meters and which rotated about its longitudinal taxis at 6 revolutions per minute. The indicated quantities of lump glass and liquid components (including the thin liquor) were introduced into the reactor under normal pressure through an inlet opening before the reaction mixture was heated, after which the reactor was closed, set rotating and heated.

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A 25 m capacity static vessel equipped with a siege plate was used for Examples 6 to 8. In this type of reactor, the lump glass rests on the sieve plate and the liquid phase is passed -through it by pump-recirculation.
In Examples 6 and 7, all the components were introduced in-to the reactor under normal pressure before heating of the reaction mixture, in the same way as before.
In Example 8, however, a different procedure was adopted in that the water glass solution was initially prepared with demineralized water and without the addition of thin liquor.
The thin liquor was then pumped into this solution in the reactor without intermediate cooling and against the pressure of the reaction mixture. Accordingly, a considerably larger quantity of solid glass was introduced at the outset. After 60 minutes' dissolution (at 140 to 145C), a water glass soul-lion containing 7.5% of NATO and 25.1~ of Sue was obtained in this way. After the thin liquor had been pumped in, a further reaction time of 120 minutes was allowed, as in the other Examples, for the dissolution of lump glass.
Example 1 (Comparison Example) shows that, even where very dilute liquor containing only 4% of Noah is used as the . "dissolving water", there is a significant volubility limit in the absence of dissolved silicate. The wa~erglass solution obtained after 120 minutes has a technically unacceptable low . 25 solids content. The volume/time yield is extremely low (coy-.pared with the first dissolving stage of only 60 minutes dune-lion in Example 8).
Accordingly, Examples 2 to 8 clearly illustrate the ad-vantage of the process according to the invention.
All the percentages quoted are percentages by weight.

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_ . ..._.___ _._ ._ The proceeding specific embodiments are illustrative of the practice o-E the invention. It is to be understood however that other expedients known to those skilled in the , art or disclosed herein may be employed without departing I from the spirit of the invention or the scope of the append- ¦
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Claims (4)

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:

1. In the process for the production of aqueous sodium silicate solutions containing at least 23% by weight of SiO2 and having a ratio by weight of SiO2 to Na2O of from 2.0 to 3.0 comprising dissolving solid sodium silicate glass lumps having a ratio by weight of SiO2 to Na2O of from 3.0 to 3.5, in alkali-free water at 135° to 155°C and under the corresponding autogenous vapor pressure of the water in a first dissolving step until an aqueous sodium silicate solution containing at least 20% to at most 30% by weight of SiO2 is obtained, adjusting the alkali content and recovering said aqueous sodium silicate solutions, the improvement consisting essentially of adding thin liquor having a content of between 5% and 20% by weight of NaOH to said aqueous sodium silicate solution con-taining at least 20% to at most 30% by weight of SiO2, in such a quantity as to adjust the alkali content to produce ratios by weight of SiO2 to Na2O of from 1.8 to 2.6 in the mixture of the two components with a solids content dependent upon the concentration of said thin liquor, dissolving more of said solid sodium silicate glass lumps in this mixture in a second dissolving step at 135° to 155°C and under the corresponding autogenous vapor pressure until a concentration of at least 23%
of SiO2 in solution is reached, with a ratio by weight of SiO2 to Na2O of from 2.0 to 3.0, and recovering said aqueous sodium silicate solution.

2. The process of claim 1, wherein said first dissolving step is conducted at a temperature of between 140° and 150°C, said thin liquor is added under pressure to said silicate product of said first dissolving step at a temperature of between 140° and 150°C and the corresponding autogenous pressure, and said second dissolving step is conducted at a temperature of between 140° and 150°C.

3. The process of claim 1, wherein said temperature is maintained by insertion of steam at about

4 bars.