HK1029568A - Process to obtain a high-concentration colloidal silica suspension and product thus obtained - Google Patents

Description

Process for the preparation of a highly concentrated colloidal silica suspension and product obtained by this process

Technical Field

The invention relates to a method for obtaining colloidal silica suspensions in high concentrations, in particular in concentrations above the corresponding gel point. The invention also relates to the products obtained by this process.

Prior Art

Based on SiO₂Suspensions of (a) including silica having a nanometric scale, suspensions also known as alkaline sols having a basic pH and acidic suspensions known as acidic sols are well known and find their use, for example as binders for high temperature precision casting.

Such suspensions obtained by conventional processes are described in GB patent 1,348,705. The preparation process disclosed therein is based on an ultrafiltration step, which is strictly controlled, in particular with regard to the salt content, to prevent irreversible gelling phenomena.

A process for obtaining colloidal silica by three successive reactions is described in us patent 5,458,812. In this process too, the final suspension is concentrated by ultrafiltration.

Both of the above-mentioned methods can produce colloidal silica suspensions having dispersed particle sizes on the order of tens of nanometers. In fact, in the above process, given the average size of the water-dispersed silica particles, there is a limit in the concentration, called the gel point, below which the gelling phenomenon occurs spontaneously as the silica particles aggregate. The above data are well known (R.K. IIer, the chemistry of Silica (Silica chemistry), John Wiley&Sons, NY) demonstrated smaller particle sizes, theseThe lower the concentration at which spontaneous gelation of the particles occurs. The values reported at a pH of about 9.5 are as follows: average particle size gel point specific surface (nm) (% wt) (m)²/g)

5 8-20 550

7 33-36 392

12 45-50 229

22 55-60 125

Wherein the range of said specific surface area generally corresponding to the particle size is obtained by the following formula: specific surface area (m)²(ii)/g) 2750/particle size (nm) (cited in reference)

Therefore, after setting the particle size, it is impossible to obtain a concentration higher than the corresponding gel point.

Specific operating conditions capable of solving the disadvantages in the prior art have been found.

SUMMARY

It is therefore an object of the present invention to provide a method for obtaining colloidal silica-based sols having a concentration above the corresponding gel point.

It is a further object of the invention to produce an alkaline suspension based on colloidal silica in which the concentration of colloidal silica is greater than the corresponding gel point.

A further object of the invention is the use of such an alkaline suspension based on colloidal silica as a binder in precision casting, the concentration of silica in such a suspension being greater than the corresponding gel point.

Other objects of the present invention will become apparent from the detailed description of the invention. Detailed Description

In this specification, all percentages are expressed in weight percent unless otherwise indicated.

A stable starting suspension which can be used in the process of the invention can be obtained by: by making aqueous solutions of alkali metal silicates, e.g. in which the alkali metal silicate may be SiO₂/Na₂The ratio of O is approximately equal to 3.36: 1 (about 5-6% by weight SiO in solution)₂) The sodium silicate (D) is passed through a RELITE  CF strong cation exchange column (Resindion corporation) at room temperature to obtain unstable silicic acid (acid sol) having a final pH of 2 to 3.5.

The acid sol was stabilized at room temperature as follows. The solution is stirred vigorously and alkali silicate is added rapidly to change the pH value from about 2 at the beginning to 9-11 at the end.

E.g. about 10m³The acid sol of (1) can be added to about 800 liters of 20-30% sodium silicate solution by weight to achieve a basicity of about 140-160 meq/liter of the original acid sol.

The effect of this rapid addition of base is to move it away from the critical pH region where spontaneous gelling can occur as quickly as possible.

This stabilized suspension was charged to a suitable evaporator and heated. The concentration is carried out by boiling under vacuum at a temperature of about 90-98 deg.C, preferably about 95 deg.C, until SiO₂To a final concentration of about 5-15% by weight, preferably about 6% by weight, a pH of 9-10 and a particle size of less than 10nm, preferably in the range of 2-5 nm.

After the desired concentration has been obtained, the suspension is subjected to cold ultrafiltration so that it can be cooled from the boiling temperature to room temperature as quickly as possible, for example using a suitable heat exchanger. In order to be able to obtain the concept of the time required for the cooling operation, it was noted that the temperature was reduced to 15-30 ℃ within a few hours.

For ultrafiltration, membranes well known to those skilled in the art, such as planar membranes or crimped membranes having a cut-off (cut) of 10,000 to 30,000 daltons, may be conveniently used.

Since the suspension heats up as it passes through the film, it is recommended that the control temperature should not exceed 50 ℃ or that the cut-off value changes, i.e. becomes large.

After ultrafiltration as described above, the starting suspension is concentrated, but the particle size of the starting particles remains unchanged: 2-5nm, specific surface area of 500-²(ii) in terms of/g. Subsequently, once the ultrafiltration has been completed, a suspension having a silica particle size of less than 10nm and a concentration of more than 50% by weight, preferably more than 30% by weight, is obtained.

After ultrafiltration, the content of cations and anions concentrated by the ultrafiltration membrane must be adjusted. This ion-conditioning step is quite important; in fact, if this step is carried out sufficiently, it will lead to instability of the suspension and gelling.

Techniques known to those skilled in the art may be used for this adjustment, for example using ion exchange resins.

Within the scope of the invention, this adjustment can be carried out in a conventional manner, as shown below. A certain amount of the product, preferably 20-50% of the total amount, is passed through a strong cation exchange resin layer that can trap sodium ions, such as with the aid of the limit company's RELITE  CF. The acidic sol thus obtained is mixed with the rest of the product. Then, a certain amount, preferably 20 to 50% of the total amount, is taken out of the resulting mixture and passed through a layer of weak anion exchange resin, for example with RELITE  4MS from Resindion. The eluate and the rest of the product are then mixed.

The specific surface area of the final product and the product obtained according to the invention is 400-600 m²Per g, preferably 527 to 550m²The final concentration of stabilizing ions corresponding to the silica particle size and concentration in g is shown below:

sulfate ion: 20 to 400mg/l, preferably 100 to 170mg/l

Sodium ion: 0.10 to 0.80 wt.%, preferably 0.27 to 0.40 wt.%.

The concentrated solutions of the present invention have wider applications than corresponding low concentration solutions due to their high concentration. It is in fact well known that colloidal silica can be used as a flocculating agent, for example in the food industry instead of bentonite, or as an additive in the plastic material and paper industry.

The suspension of the invention is particularly effective in the clarification of white wine and fruit juices and in increasing the yield of the clarified product, due to its higher concentration.

In the field of plastics for food applications, the suspensions according to the invention, due to their large specific surface area, are of great interest for use as osmo-pressure regulators which are more effective than the known products.

With respect to papermaking, the suspensions of the present invention are particularly advantageous in providing liquid retention.

In particular, the use of the concentrates of the present invention as water-based binders with little environmental impact is advantageous for investment casting applications.

The following examples are illustrative and do not limit the scope of the invention. Example 1 colloidal silica preparation Process details

Silicate glass (SiO)₂/Na₂O ratio of 3.36) and an amount of water sufficient to obtain a final silicate concentration of 25% by weight, in an autoclave at a pressure of 5 to 6 atmospheres. The solution was cloudy due to the presence of clay generated from siliceous sand, the solution was decanted and transferred to a flocculation tank where it was further diluted to about 20% with water. The solution was then heated to about 70 ℃ with stirring and the low molecular weight cationic coagulant was added, stirred at this temperature for about 2 hours, and then decanted for 48 hours. The recovered purified liquid is further diluted with waterTo about 5%. The diluted solution is passed through an exchange column packed with strong cationic resin RELITE  CF to facilitate the exchange of sodium silicate with resin hydrogen ions to obtain unstable colloidal silicic acid (acid sol). 10m³The acid sol is put into a tank equipped with a propeller stirrer and stirred vigorously. The pH was raised from 2 to around 10 by the rapid addition of alkali silicate (800 liters). The stabilized solution is heated to boiling (95 ℃ C.) under vacuum until SiO₂The final weight concentration of (A) is about 6%, the pH value is about 9.5, and the particle size is 2-5 nm. The suspension was then cooled from 95 ℃ to room temperature over a period of several hours and subjected to ultrafiltration. Ultrafiltration uses a planar membrane with a cut-off of 20,000 daltons. Once the ultrafiltration was complete, a suspension of silica having an average particle size of 2.6nm was obtained.

After ultrafiltration, the cations and anions in the suspension were adjusted to remove 30% of the total product and passed through a strong cation resin bed, RELITE  CF, which captures sodium ions. The acidic sol thus obtained is mixed with the rest of the product. 30% of the total amount of these mixtures was taken through a layer of weakly anionic resin RELITE  4 MS. The eluate is then mixed with the remaining mixture.

The final product obtained in this way has the following characteristics:

the weight concentration of silica was 25%;

surface area of 530m²/g；

The particle diameter was 2.6 nm.

The test specimen K mentioned below was obtained by this method and was used for the continuous test. Example 2 stability test. A quantity of sample K having an initial temperature of 20 ℃, a density of 1.14 g/ml, a viscosity of 11 seconds (Ford cup) B4) and a pH of 1O was held at a temperature of 60 ℃ for 20 days. This corresponds to a product which has been left to stand at room temperature for 200 days and which has a density of 1.13 g/ml at 60 ℃ and a viscosity of 11 seconds (Ford cup B4) and a pH of 10. After a further 20 days at 60 ℃ no significant changes occurred, demonstrating the high stability of this product. EXAMPLE 3 preparation of ceramic Shell mold for precision casting and its Properties

The test was carried out on sample K and on another control sample not subjected to ultrafiltration, hereinafter referred to as A, which has the following characteristics: the density is 1.2kg/dm³，SiO₂Has a weight concentration of 30%, a pH value of 9.5 and Na₂O accounts for 0.30%, the particle diameter is 12nm, and the specific surface area is 300m²(ii)/g, the chloride ion concentration is 50ppm, and the sulfate ion concentration is 450 ppm.

The selection of the comparative samples was based on the following criteria: the product chosen must have a specific surface area (at least 500 m) that is greater than that of the product to which it is compared²/g) small specific surface area, with a high silica content. The effect of these two physical quantities on the mechanical resistance and permeability can be observed in this way.

The purpose of this was to demonstrate how the increase in specific surface area compensates for the reduction in silica content (with a concomitant reduction in overall product costs) and at the same time guarantees not only mechanical properties (modulus of rupture MOR at room temperature and at high temperature) which meet the industrial standards, but also an increase in permeability at room temperature and at high temperature (900 ℃).

The preparation process of the shell mold is based on the following process:

-preparing the wax component by pressure injection of a pressurized liquid synthetic material into the mould.

-assembling the wax parts by welding, so as to obtain a composite comprising the parts to be prepared and, if present, the parts to be prepared

Ceramic coated high temperature material connects the assembly (module) of part-channel (gate).

-coating the mould set with ceramic material by alternately dipping the mould set a few times into an immersion liquid comprising a binder and ceramic material and spraying a refractory sand.

The above operations are used to prepare ceramic shell molds which finally require drying under controlled humidity and temperature conditions.

Dewaxing in an autoclave at a maximum pressure of 10 atmospheres to remove the wax from the ceramic shell mould. The shell molds thus obtained are referred to as shell K and shell a.

For comparison purposes, the immersion liquids used in the manufacture of ceramic shell molds for casting were manufactured using samples K and A. The production cycle, which varies depending on the intermediate drying time and the relative conditions (whether or not aeration is applied), is shown in table 1.

TABLE 1

	Adhesive A	Adhesive K
			Intermediate drying for 1 hr without aeration	A1	K1
1 hour intermediate drying, ventilating	A1V	K1V
			n hours intermediate drying without aeration	An	Kn
n hours intermediate drying, ventilating	AnV	KnV

For viscosityCocktail of a and K, primary and secondary infusions were prepared as shown in table 2.

TABLE 2

Adhesive agent	Adhesives A and K
		Primary infusion powder	Zircosil-200 mesh
Primary impregnation liquid/binder ratio	4.5kg/l
		Defoaming agent	1% octanol
Wetting agent	0.5％SYNPERONIC
		Viscosity (initial infusion)	90 seconds (Ford cup B4)
Secondary leaching solution powder	Calcined china clay-200 meshes
		Secondary leach liquor/binder ratio	2kg/l
Viscosity (Secondary infusion)	30 seconds (Ford cup B4)

Ceramics were prepared from the above-described immersion liquids according to the procedures shown in Table 3And (5) shell molding. The firing and casting conditions for the shell molds obtained with binders a and K are listed in table 4. The shell mold thus obtained was subjected to mechanical testing to obtain a modulus of rupture MOR at room temperature and 900 ℃. The test was performed on a neitssch tester 422S.

TABLE 3

Spraying sand grains
			Number of initial layer	1	Zircon 80/120
Number of next layer	33+ finishing	Calcined china clay 30/80 calcined china clay 30/80
			Intermediate drying	1 to 4 hours
Final drying	24 hours
			Drying conditions	45% +/-5% UR21.5 +/-1.5 ℃ with/without ventilation
Dewaxing in an autoclave	8 atm for 10 min

TABLE 4

	Shell mould A	Shell mould K
			Shell mould casting (identification)	A1(2) and A1V	K1(2) and K1V
Test casting	3	3
			Preheating conditions	1150 ℃ for 1 hour	1150 ℃ for 1 hour
Type of alloy	AISI 316	AISI 316
			Casting temperature	1550℃	1550℃
Remarks for note	Integral casting	Integral casting
			Surface quality	Good taste	Good taste

The high temperature test adopts the following steps:

dewaxing of the test specimens in an autoclave

Calcination to 1000 ℃ without intermediate pauses

Low temperature reduction in the furnace

Placing the sample in the apparatus at room temperature

The sample is placed in the apparatus and heated to 900 deg.C

Bending test at high temperature

Tests carried out at room temperature found that the modulus of rupture values of the shell molds obtained with binder a (with/without forced ventilation) were, on average, higher than those obtained with binder K. (Table 5-modulus of rupture at room temperature and 900 ℃ in MPa)

TABLE 5

Temperature of	At room temperature				900℃
							Intermediate drying time	1	2	3	4	1	4
A	6.63	6.86	6.56	8.57	14.65	15.20
							AV	6.29	7.40	6.21	7.26	14.65	14.35
K	5.12	5.86	6.15	6.77	15.05	14.45
							KV	5.60	5.17	5.62	5.68	13.05	14.90
Comparative values in industry	5	5	5	5	12	12

In any case, the values in both cases are greater than 5MPa, 5MPa being considered as the lowest reference value in the foundry.

The values in terms of heat resistance (after intermediate drying for 1 hour and 4 hours) (Table 5) are comparable to the reference values, but higher than 12MPa for the industrial data.

It is to be noted that the modulus of rupture is a measure of the stress to which the sample is subjected; this measurement is largely related to the thickness of the shell mold. However, the overall resistance to failure is given by the tensile strength. (see Table 6-tensile Strength in Newton (N) at Room temperature)

As can be seen from Table 6, on average, shell mold K is more resistant to failure than shell mold A. The higher resistance is manifested by a higher tensile strength, which is strongly influenced by the occurrence of cracks in the ceramic shell mold.

TABLE 6

Ventilation time and Condition (hours)	A	K
			1	31	25.8
1V	25.4	29
			2	31.3	41.7
2V	32.3	28.3
			3	32.4	35.7
3V	23.8	28
			4	37.5	49
4V	28.3	28.5

Permeability tests at room temperature and 900 ℃ were carried out on the obtained shell molds. The test was performed according to the standard ICI 775-83.

Permeability testing is generally done to predict the behavior of shell molds during the wax removal process and to understand their filling characteristics. Good permeability at room temperature is consistent with good shell mold performance in the paraffin removal process (no cracks); but it is an indication of the alloy filling the ceramic mold at high temperatures.

The data of the comparative tests of samples a and K may prove that at room temperature the two products show almost the same characteristics, whereas at high temperatures (900 ℃) the products of the invention are absolutely better. (see Table 7-Permeability test at room temperature and 900 ℃).

TABLE 7

Test specimen	Permeability at room temperature (10)-5)	Permeability at 900 ℃ (10)-5)
			A3/1	84.9	6.29
A3/2	83.5	6.96
			A3/3	92.7	6.55
A3/4	99.3	7.55
			A2V/1	103	10.4
A2V/2	96.4	6.82
			A2V/3	88.2	6.24
A2V/4	94.6	6.39
			A2V/5	103	8.79
K3/1	178	20.6
			K3/2	204	28.8
K3/3	188	25.5
			K3/4	197	25.5
K3/5	200	29.1
			K2V/1	169	28.1
K2V/2	171	24.8
			K2V/3	184	26.2
K2V/4	185	26.2
			K2V/5	164	20.6

Claims (16)

1. A process for the preparation of a colloidal silica suspension having a concentration above its gel point comprising the steps of:

-preparing a stable suspension by passing an aqueous solution of an alkali metal silicate through a strong cation exchange column at room temperature to obtain an unstable silicic acid having a final pH value of 2-3.5;

-stabilizing the acid sol at room temperature by vigorous stirring and rapid addition of alkali silicate to bring the final pH value to 9-11 rapidly;

-charging the stable suspension to an evaporator and at about 90-9%The suspension was concentrated under vacuum at 8 ℃ until SiO₂The final concentration reaches about 5-15% wt, the pH value is 9-10, and the particle size is less than 10 nm;

subjecting the suspension to cold ultrafiltration, wherein the suspension is cooled as quickly as possible from the boiling temperature to room temperature or below.

2. The process according to claim 1, wherein the silicate is SiO₂/Na₂Sodium silicate having an O ratio equal to about 3.36: 1.

3. The method according to claim 1, wherein in the ultrafiltration step, the cooling takes about two hours.

4. The process according to claim 1, wherein in the ultrafiltration step, cooling is carried out until the temperature is 15 to 30 ℃.

5. A process according to claim 1, wherein ultrafiltration is carried out on a membrane having a cut-off of 10,000 to 30,000 daltons.

6. A process according to any one of claims 1 to 5, wherein the temperature is controlled to be kept below 50 ℃ during the ultrafiltration step.

7. A process according to any one of claims 1 to 6, wherein the ultrafiltration step is followed by a sol stabilization step by: passing 20-50% of the total amount of the suspension through a strong cation exchange resin layer capable of capturing sodium ions; the resulting acidic sol is mixed with the rest of the suspension; then, 20 to 50% of the total amount is taken out of the obtained mixture and passed through a weak anion exchange resin layer, and the eluate is mixed with the rest of the mixture.

8. A process according to claim 7, wherein the particle size and silica are mixedThe final stable ion concentrations at consistent concentrations were as follows: the specific surface area of the material is 480-600 m²Suspension/g, sulfate ion: 20-400 mg/l, sodium ion: 0.10-0.80% by weight.

9. The process according to claim 7, wherein the final stable ion concentration consistent with the particle size and silica concentration obtained according to the invention is as follows: the specific surface area is 527 to 550m²Suspension/g, sulfate ion: 100-170 mg/l, sodium ion: 0.27-0.40 wt%.

10. A colloidal silica alkaline suspension having a concentration above its gel point, wherein silica particles having a size of less than 10nm are contained in a concentration of more than 50% by weight.

11. A colloidal silica alkaline suspension having a concentration above its gel point, wherein silica particles having a size of less than 10nm are contained in a concentration of more than 30% by weight.

12. Use of a suspension according to any of claims 10 and 11 as a flocculant.

13. Use of a suspension according to any of claims 10 and 11 as a food flocculant.

14. Use of the suspension according to any one of claims 10 and 11 as an additive to plastics for food use.

15. Use of the suspension according to any of claims 10 and 11 as an additive in papermaking.

16. Use of a suspension according to either of claims 10 and 11 as a water-based binder for investment casting technology with little environmental impact.