AN INORGANIC BINDER SYSTEM FOR FOUNDRIES

Description

[0001] The object of the present invention is a powder composition that acts as an activator in foundry binder systems comprising an aqueous solution of sodium silicate.

[0002] The powder composition of the invention comprises sodium silicate having CAS Registry Number 1344-09-8 and a SiO₂/Na₂O molar ratio comprised between 3.0 and 3.6, preferably between 3.2 and 3.5, and sodium aluminosilicate having CAS Registry Number 1344-00-9. Optionally, the powder composition of the invention may furthermore comprise additives.

[0003] Objects of the invention furthermore are the binder systems comprising the powder composition of the invention and aqueous solutions of sodium silicate, optionally comprising additives, the use of said binder systems for the production of foundry moulds, such as cores and/or moulds, and likewise the moulding material mixtures for the production of foundry cores and/or moulds comprising the binder systems of the invention and the silica sand and/or other granular refractory material. Furthermore objects of the invention are a process for the production of said foundry moulds, such as cores and/or moulds, comprising a production step of the moulding material mixtures comprising the binder systems of the invention, together with silica sand and/or other granular refractory material, and a heating step of said moulding material mixtures, and the cores and/or moulds obtainable with said process.

[0004] The foundry moulds, such as cores and /or moulds into or around which the molten metal will be poured, are comprised of a granular refractory material, such as, for example silica sand, mixed with a binder system, normally composed of two or more components.

[0005] The foundry moulds must meet various requirements. In particular, alongside sufficient stability and thermal resistance to accommodate the molten metal, said moulds must furthermore feature stability against moisture present in the processing environment, so as to be usable even some time after the production thereof and after being subjected to the mechanical stress of transportation from the place of production to the place of use thereof.

[0006] The majority of foundry binder systems generally used are of organic nature, since organic mixtures allow the production of binder systems with a far superior performance to those of the inorganic-based binder systems currently available.

[0007] In the organic-based binders, i.e. those comprising organic resins as binders, the hardening reaction which binds the inert material is accelerated by a chemical catalyst in a liquid or vapour form.

[0008] Nevertheless, despite the excellent performance offered by organic binder systems, the entry into force of stricter environmental regulations than in the past about acceptable emission levels in the workplace for workers in the industry and in the atmosphere, has revived interest in inorganic foundry binders, such as binder systems based on alkali silicate and, in particular, sodium silicate in an aqueous solution.

[0009] Therefore, the need for totally inorganic-based binder systems is given by the need for binder-based moulds that do not emit hazardous (i.e. harmful, toxic, corrosive, irritant, etc) substances, such as - for example - benzene, toluene, xylene, phenol formaldehyde, when the high-temperature molten metal is poured into (or around) the mould, resulting in the breakdown of the said mould.

[0010] It must furthermore be considered that, in the case of the permanent-mould casting, i.e. casting poured in a permanent metal mould, the organic binder systems deposit a layer of a tarry substance on the metal surfaces, which leads to frequent cleaning, and as a result, a slowdown in production.

[0011] In these cases, the inorganic binder systems - which should only develop water as a result of the heat - can prevent this slowdown in production.

[0012] Although well known in the industry for some time, interest in aqueous solutions of sodium silicate for use in the production of foundry moulds had decreased due to the limited performance of the moulds produced using the binder systems comprising such solutions, caused by the poor moisture resistance which meant they had to be used immediately after production and could not be stored for later use.

[0013] Usually, the aqueous solutions of alkali silicates are obtained by melting the silica with an alkali carbonate and dissolving the obtained product in water.

[0014] Sodium silicate is the most commonly used alkali silicate, above all for cost reasons. Only in special cases outside the foundry industry potassium silicate is used, for example, in binders for the farming industry, where sodium ion is an undesirable component. Lithium silicate is used in very specific applications, for example for sealing concrete surfaces.

[0015] Aqueous solutions of alkali silicates are characterised by the following parameters:

• SiO₂/Me₂O molar ratio (or module), in which 'Me' stands for metal. When Me is sodium, i.e. in the case of sodium silicate, the weight ratio SiO₂/Na₂O corresponds, reasonably approximately, to the molar ratio and is routinely used instead of the latter (in the case of sodium silicate, the weight ratio can be converted to the molar ratio by multiplication by 1.032).
  The aqueous solutions of sodium silicate currently available on the market usually have a molar ratio of between 2.0 and 3.5. The choice of the most suitable type depends on the specific needs, bearing in mind that the reactivity of the sodium silicate increases proportionally to the molar ratio value;
• density and viscosity, which are linked, in equal molar ratio, to the amount of water contained in the solution.

[0016] For use as a binder for foundry sands, the aqueous solution of alkali silicate is mixed with the sand and any other optional components/additives: the resulting moist mixture is placed into the core box or in the flask where, depending on the process used, it hardens by effect of the heat and/or a chemical reaction with an external agent.

[0017] The choice of the type of aqueous solution of alkali silicate to be used as a foundry binder for moulding materials must reconcile the following conflicting requirements:

• good flowability of the mixture with the sand in order to guarantee good compaction during moulding and consequently more compact cores and moulds. The flowability is enhanced by low-viscosity alkaline silicate solutions, i.e. those with a rather high water content;
• a good bench life of the sand/alkali silicate mixture: the mixture should not harden too quickly due to spontaneous evaporation, the carbon dioxide present in the air, or the presence of any additives. The bench life of the mixture is therefore enhanced by alkaline silicate solutions with a rather high water content and a rather low molar ratio, i.e. close to the lower limit stated above;
• high reactivity of the sand/alkali silicate mixture after being compacted in the core box or in the flask in order to ensure high production rhythms. The reactivity of the mixture is enhanced by an alkaline silicate with a low water content and with a molar module close to the upper limit stated above.

[0018] Over time, research has been carried out into different ways of bringing about the hardening of aqueous solutions of sodium silicate, either by chemical means or by thermal means, to allow the use of said solution as a binder for sand and/or other refractory materials for use in foundry moulds, without - however - managing to produce moulds offering long-lasting moisture resistance.

[0019] On page 70 of the book "Anorganische Binder zur Form- und Kernherstellung in der Giesserei", published by Schiele & Schön, in 2012 and herein incorporated by reference, mention is made of a Czechoslovakian patent dated 1947, registered to L. Petrzela, which describes a process for cold-hardening moulds and cores bonded with aqueous solutions of sodium silicate, which uses carbon dioxide. This process is still used in foundry, albeit limited.

[0020] For some time, also in Italy, a Japanese process has been used which is known as the Nishiyama process, dated 1961; said process involves the use of ferrosilicon powder for the cold hardening of casting moulds bonded with sodium silicate solutions. This process, which is described, for example, on p. 1066 of the volume "Giesserei Lexikon", 1986 edition, Fachverlag Schiele & Schön GmbH, herein incorporated by reference, was abandoned because of the risks associated with the generation of hydrogen during the hardening reaction.

[0021] Another process used in the past involved the use of blast-furnace cements for the cold hardening of foundry moulds bonded with aqueous solutions of sodium silicate ("Use of blast furnace slag cements as hardening agents for silicate-bonded moulding sands", Donald W. and D. Smith, The British Foundryman, January 1968, p. 34-40, herein incorporated by reference).

[0022] One process still used, albeit in a limited way, involves the use of liquid esters, mostly from acetic acid, for the cold-hardening of moulds and cores bonded with aqueous solutions of sodium silicate ("Recent developments in the silicate-based foundry processes", D. Warren, The British Foundryman, December 1971 p. 449-455, herein incorporated by reference).

[0023] The above listed processes are not widespread and are limited to the production of solely low-strength articles and prone to strong decay due to ambient humidity.

[0024] An improvement in performance was achieved by reducing the water content of the articles after hardening, thereby making it possible to improve the prospects for use of the aqueous solutions of silicate sodium in foundry.

[0025] Patent US4226277, herein incorporated by reference, describes a foundry binder system consisting of an aqueous solution of sodium silicate, in which a flow of air, optionally heated, is forced through a mass of sand bonded with said system to accelerate the evaporation of the water contained in the sodium silicate solution and, consequently, accelerate the hardening thereof.

[0026] Patent application WO98/49118, herein incorporated by reference, describes the controlled gelification and subsequent hardening of the sodium silicate solutions for use as adhesives, in particular for outdoor applications when weather-resistance is required. The hardening of said solutions is brought about by adding additives such as amorphous silica or an amorphous metal silicate, chosen from magnesium silicate, calcium silicate or aluminium silicate.

[0027] Lastly, patent application EP1802409, herein incorporated by reference, describes a foundry moulding mixture comprising a granular refractory material, a sodium silicate solution and a hardener consisting of a metal oxide in particulate form, chosen from silica, alumina, titanium oxide, and zinc oxide.

[0028] There is a considerable need, therefore, for alternative foundry binder systems based on alkali silicate in an aqueous solution, in particular sodium silicate, for the production of foundry moulds, which offer good strength and are not subject to decay during storage in a humid atmosphere.

[0029] Surprisingly, an inorganic powder composition has now been found which is particularly suitable for hardening the mixtures of moulding material comprising aqueous solutions of sodium silicate and silica sand and/or other granular refractory material. Indeed, foundry moulds, such as the cores and/or moulds obtained with the binder systems and/or moulding material mixtures of the invention, feature high moisture resistance during storage, thus allowing later use thereof, some time after the production thereof, including after the transportation from the place of production to other places for the subsequent use thereof.

[0030] The binder systems and the moulding mixtures of the invention are totally inorganic-based and therefore furthermore meet recent environmental safety standards requiring limited release of hazardous and/or toxic substances into the environment. The object of the invention is therefore a powder composition comprising sodium silicate (CAS Registry Number 1344-09-8) and a SiO₂/Na₂O molar ratio comprised between 3.0 and 3.6, preferably between 3.2 and 3.5, and a sodium aluminosilicate powder (CAS Registry Number 1344-00-9).

[0031] Preferably, the sodium silicate powder has an average grain size comprised between 60 and 100 µm, preferably between 70 and 90 µm, and still more preferably of 80 µm, and/or the sodium aluminosilicate powder has an average grain size comprised between 2 and 15 µm, preferably between 5 and 10 µm, and still more preferably of 7 µm.

[0032] Sodium silicate powder is scarcely soluble in cold water, but becomes soluble when warm. Therefore, this component does not negatively affect the storage potential (bench life) of the moulding material comprising said component, since, thanks to its high molar ratio, it acts as an accelerator during the subsequent heating process (baking).

[0033] The sodium aluminosilicate powder is insoluble in water.

[0034] According to a particularly preferred aspect, the powder composition of the invention comprises 60 to 90% by weight of sodium silicate with a SiO₂/Na₂O molar ratio comprised between 3.0 and 3.6, preferably between 3.2 and 3.5, and 10 to 40% by weight of sodium aluminosilicate, wherein said percentages refer to the total weight of the powder composition.

[0035] Furthermore, the powder composition may comprise additives.

[0036] According to a preferred aspect of the present invention, additives are understood as substances and/or compounds which are useful for improving the flowability of the moulding material mixtures comprising the powder composition of the invention, such as for example mica and/or silver graphite, substances and/or compounds which are useful for improving the surface of the castings, such as - for example - iron oxides, or substances and/or compounds which are useful for improving the breakdown (shake-out) of cores or moulds after casting. In the latter case, carbohydrates are used, such as sucrose, cereal flours, coke powder, and/or wood powder.

[0037] According to a preferred aspect, the powder composition of the invention comprises 1 to 25% by weight of additives, preferably 5 to 15 % by weight of additives, wherein said percentages refer to the total weight of the powder composition of the invention. The powder composition of the invention is obtained by mixing sodium silicate and sodium aluminosilicate and any optional additives, as defined above, in a powder mixer, preferably of the conical type, which is activated until a homogeneous powder is obtained.

[0038] The powder composition of the invention enables the hardening of a moulding material mixture comprising aqueous solutions of sodium silicate and sand and/or other granular refractory material, thereby allowing foundry moulds to be obtained, such as cores and/or moulds, which are safe for the environment and for the workers in the industry, feature resistance to ambient moisture, and which extend the storage potential for use thereof for some time, subsequently to the time of production.

[0039] The object of the invention, therefore, is a binder systems comprising the powder composition of the invention, as defined hereinabove, and an aqueous solution of sodium silicate.

[0040] According to a preferred aspect, the aqueous solution of sodium silicate comprises 25 to 50% by weight of sodium silicate, more preferably, 35 to 45% by weight of sodium silicate, and between 50 to 75% by weight of water, wherein said percentages refer to the total weight of the aqueous solution of sodium silicate. According to a further preferred aspect, the aqueous solution of sodium silicate has a SiO₂/Na₂O molar ratio comprised between 1.9 and 2.5, preferably between 2.0 and 2.4, and more preferably between 2.1 and 2.3, and/or the aqueous solution of sodium has a density at 20°C comprised between 1.30 and 1.50 kg/dm³, preferably comprised between 1.35 and 1.45 kg/dm³.

[0041] Particularly preferred are the aqueous solutions of sodium silicate which have a SiO₂/Na₂O molar ratio comprised between 2.1 and 2.3, with a density at 20°C comprised between 1.35 and 1.45 kg/dm³.

[0042] The aqueous solution of sodium silicate is prepared by taking an aqueous solution of sodium silicate having a molar ratio within the above cited range easily available on the market, and appropriately diluting it with water in a mixer equipped with a stirrer, with no need of heating.

[0043] Furthermore, the aqueous solution of sodium silicate may furthermore comprise additives.

[0044] Preferably, the aqueous solution of sodium silicate comprises 0.1 to 10% by weight of additives, wherein said percentages refer to the total weight of the aqueous solution of sodium silicate.

[0045] According to a preferred aspect of the present invention, additives are understood as substances and/or compounds which are useful for improving the flowability of the moulding material mixture, for example sodium 2-ethylhexyl sulphate, which is preferably added in aqueous solution at 40% by weight, in an amount comprised between 0.1% and 1.0% by weight, preferably between 0.2% and 0.6% by weight, with respect to the total weight of the aqueous solution of sodium silicate, or monophenyl glycol, which is preferably added in an amount comprised between 0.5% and 5.0% by weight, preferably between 1.0 and 4.0% by weight, with respect to the total weight of the aqueous solution of sodium silicate.

[0046] The aqueous solution of sodium silicate may furthermore comprise additives which improve bonding with the grains of refractory material with which said aqueous solution of sodium silicate is mixed and which improve the storage potential of the manufactured articles in a humid environment, such as, for example, silane compounds. Silane compounds of particular interest are, for example, 3-aminopropyltriethoxysilane, 3-aminopropylmethyldiethoxysilane, 3-(diethoxymethyl-silyl)propylamine, 3-glycidoxypropyltrimethoxysilane and mixtures thereof, which are preferably added in amounts of between 0.1% and 1.0% by weight, preferably between 0.2 and 0.6% by weight, with respect to the total weight of the aqueous solution of sodium silicate.

[0047] The aqueous solution of sodium silicate may furthermore comprise additives which facilitate the breakdown (shake-out) of the cores and moulds after casting, said additives usually being carbohydrates, such as sucrose, which is preferably added in an amount comprising between 1.0% and 10.0% by weight, preferably between 3.0% and 6.0% by weight, with respect to the total weight of the aqueous solution of sodium silicate.

[0048] At room temperature, whereby the term "room temperature" means a temperature comprised between approximately 0 °C and + 40 °C, preferably between +15°C and + 30°C, the aqueous solution of sodium silicate as described above is very fluid and scarcely reactive and it provides the moulding mixture containing it, i.e. to the mixture comprising the aqueous solution of sodium silicate, the powder composition, and the sand and/or other refractory material, with good flowability and a long "bench life". The invention therefore relates to the use of binder systems as defined above for the production of foundry cores and/or moulds.

[0049] The moulding material mixtures, which furthermore represent an object of the invention, comprising the binder systems of the invention, as defined above, together with the silica sand and/or other granular refractory material, such as zirconium sand (zirconium silicate) or chromite, are obtained by mixing silica sand and/or other granular refractory material with the aqueous solution of sodium silicate, as defined above, and subsequently adding the powder composition of the invention. Alternatively, the moulding material mixtures of the invention can be obtained by mixing silica sand and/or other granular refractory material with the powder composition of the invention and subsequently adding the aqueous solution of sodium silicate as defined above.

[0050] Indeed, direct mixing of the aqueous solution of sodium silicate and of the powder composition of the invention comprising sodium silicate with a molar ratio SiO₂/Na₂O comprised between 3.0 and 3.6, preferably between 3.2 and 3.5, and sodium aluminosilicate, must be avoided due to the excessive reactivity of said compounds. According to a preferred aspect, the moulding material mixtures of the invention, which if stored out of direct contact with the air, are usable for up to ten hours from the production thereof, comprise from 1.6 to 2.4 parts by weight of the aqueous solution of sodium silicate and from 0.5 to 1.5 parts by weight of the powder composition of the invention, as defined above, and 100 parts by weight of silica sand and/or other granular refractory material. According to a particularly preferred aspect, said mixtures comprise from 1.8 to 2.2 parts by weight of the aqueous solution of sodium silicate and from 0.7 to 1.2 parts by weight of the powder composition of the invention, as defined above, and 100 parts by weight of silica sand and/or other granular refractory material.

[0051] The cores and/or moulds of the invention comprise a heating step (baking) of said moulding material mixtures comprising the binder system of the invention and the silica sand and/or other granular refractory material.

[0052] The foundry cores and/or moulds thus obtainable, which furthermore represent an object of the present invention, show good strength and do not degrade during storage in humid environments.

[0053] It is an object of the invention a process for the production of said foundry cores and/or moulds, comprising a production step of the moulding material mixtures comprising the binder systems of the invention, together with silica sand and/or other granular refractory material, and a heating step of said moulding material mixtures, preferably to temperatures comprised between 150 °C and 260 °C.

[0054] According to a preferred aspect, the moulding material mixtures of the invention are placed, either manually or by means of compressed air systems, inside a core box already heated to 150 to 200 °C and/or treated with a flow of hot air at 150 to 180 °C, which quickly hardens said mixture.

[0055] The laboratory technological test results stated below clearly illustrate the advantages offered by the invention. The following examples are provided as nonlimiting illustrations of the present invention.

Examples

[0056] Control procedure utilised in the various tests.

[0057] The mixtures were prepared in the laboratory, adding first the silica sand, then the aqueous solution of sodium silicate, and finally the powder composition of the invention, using a Kitchen Aid rapid mixer (Artisan model).

[0058] The sand and the aqueous solution of sodium silicate were mixed for 1 minute and then the powder composition of the invention was added and mixed for 1 minute.

[0059] Standard identical bars were prepared with each one of the mixtures. Each mixture was compacted with five strokes of a Georg Fischer rammer PRA model in the individual core box and the resulting standard bars had a 5 cm² square breakage section.

[0060] The bars were placed on 220 x 60 mm rectangular steel plates with a thickness of 3 mm, and were baked for 6 minutes at 260 °C in a laboratory oven.

[0061] After extraction from the oven and a stay of 30 minutes at room temperature to complete cooling, the bars obtained from each of the different mixtures were divided into two groups.

[0062] The bars in the first group were stored for 24 hours in a dryer, while the bars in the second group were stored for 24 hours in a climate chamber ClimaCell 111 model manufactured by MM, which was set at 25 °C with 60% of relative humidity.

[0063] At the end of the 24 hours, flexural strength in N/cm² of all the bars was measured using a BENET tool COE model.

[0064] All the mixtures used in the examples had the LA32 French silica sand in common. Two different types of aqueous solution of sodium silicate were used, which - for the sake of brevity - shall be referred to as follows:

Solution A: aqueous solution of sodium silicate with a density of 1.42 kg/dm³ at 20° C, SiO₂ content of 25.77% and Na₂O content of 11.91%, SiO₂/Na₂O weight ratio of 2.16 and SiO₂/Na₂O molar ratio of 2.23.

[0065] Solution A, dry, after 15 minutes at 650 °C had a sodium silicate content equal to 37.68%.

Solution A1: a variant of Solution A containing additives and having the following percentage composition:

Solution A	99.3% by weight of the weight of Solution A1
Sodium 2-ethylhexyl sulphate in 40% aqueous solution	0.2%
3-glycidoxypropyltrimethoxysilane	0.5%

[0066] The sodium silicate powder used in the composition in the examples is a sodium silicate with the following chemical-physical characteristics:

- SiO2/Na2O molar ratio:	3.2-3.5
- SiO2:	61.7-66.1% by weight
- Na2O:	18.0-20.3% by weight
- loss on ignition, 800 ° C:	≤ 20% by weight
- average grain size:	80 µm

[0067] In particular, the product Britesil® C 335 was used, which is manufactured by the PQ Corporation and has the features stated above.

[0068] The sodium aluminosilicate powder used in the composition in the examples is a sodium aluminosilicate with the following chemical-physical characteristics:

- SiO2 (on ignited substance):	≥ 80% by weight
- Al (on ignited substance):	6.0% by weight
- Na (on ignited substance):	5.5% by weight
- loss on ignition:	≤ 10.0% by weight
- average particle size:	7 µm

[0069] In particular, the product Sipernat® 820 A was used, which is manufactured by Evonik and has the features stated above.

Example 1

[0070] Moulding material mixture not according to the invention:

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A;

Example 2

[0071] Moulding material mixture not according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;

Example 3

[0072] Moulding material mixture not according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;
• 0.90 parts by weight of the powder consisting of 100% by weight of sodium silicate;

Example 4

[0073] Moulding material mixture not according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;
• 0.90 parts by weight of the powder consisting of 100% by weight of sodium aluminosilicate;

Example 5

[0074] Moulding material mixture according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;
• 0.90 parts by weight of the powder composition consisting of 70% by weight of sodium silicate powder and of 30% by weight of sodium aluminosilicate powder.

Example 6

[0075] Moulding material mixture according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;
• 0.90 parts by weight of the powder composition consisting of 80% by weight of sodium silicate powder and of 20% by weight of sodium aluminosilicate powder.

Example 7

[0076] Moulding material mixture according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;
• 0.90 parts by weight of the powder composition consisting of 90% by weight of sodium silicate powder and of 10% by weight of sodium aluminosilicate powder.

Example 8

[0077] Moulding material mixture according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A;
• 0.90 parts by weight of the powder composition consisting of 68% by weight of sodium silicate powder and of 17% by weight of sodium aluminosilicate powder and of 15% by weight of coke with an average grain size of between 0 and 0.2 mm.

Example 9

[0078] Moulding material mixture according to the invention

• 100 parts by weight of French silica sand LA32;
• 2.00 parts by weight of Solution A1;
• 0.90 parts by weight of the powder composition consisting of 68% by weight of sodium silicate powder and of 17% by weight of sodium aluminosilicate powder and of 15% by weight of coke with an average grain size of between 0 and 0.2 mm.

[0079] Table 1 below provides a summary of the results obtained in Examples 1-9:

Table 1

	Flexural strength in N/cm2
	24 hours at 25°C, dry	24 hours at 25°C and 60% RH
Mixture in Example 1.	510	90
Mixture in Example 2.	565	60
Mixture in Example 3.	500	105
Mixture in Example 4.	0	0
Mixture in Example 5.	435	430
Mixture in Example 6.	470	395
Mixture in Example 7.	510	385
Mixture in Example 8.	475	400
Mixture in Example 9.	395	360

[0080] The results obtained highlighted the following:

• the aqueous solution of sodium silicate mixed with sand alone (Example 1) offers no possibility of practical use as a foundry binder, despite the excellent strength values obtained through baking: the dramatic decrease in strength caused by the storage of the finished product in a humid environment is catastrophic. Not even the incorporation of two additives (Example 2) improved the result;
• the mixing of the aqueous solution of sodium silicate and sand with sodium silicate powder having an extremely high molar ratio (Example 3) did not improve the situation: the decrease in strength caused by the moisture in the finished product remains unacceptable;
• the mixing of the aqueous solution of sodium silicate and sand with sodium aluminosilicate (Example 4) is absolutely unviable, because the sodium aluminosilicate alone is too reactive in relation to the sodium silicate and burns the mixture consisting of the aqueous solution of sodium silicate and sand;
• with the mixing of the aqueous solution of sodium silicate and sand with a powder composition according to the invention (Examples 5, 6, 7, 8, 9), the strengths obtained in the finished product with the baking are almost virtually unchanged in a humid atmosphere.

Claims

1. A powder composition comprising sodium silicate having CAS Registry Number 1344-09-8 and a SiO₂/Na₂O molar ratio comprised between 3.0 and 3.6, preferably between 3.2 and 3.5, and sodium aluminosilicate having CAS Registry Number 1344-00-9.

2. The powder composition according to claim 1, comprising from 60 to 90% by weight of sodium silicate and from 10 to 40% by weight of sodium aluminosilicate, wherein said percentages refer to the total weight of the powder composition.

3. The powder composition according to any one of the preceding claims, further comprising additives.

4. The powder composition according to claim 3, comprising from 1 to 25% by weight of additives, preferably from 5 to 15% by weight of additives, wherein said percentages refer to the total weight of the powder composition.

5. The powder composition according to any one of the preceding claims, wherein the sodium silicate has an average grain size comprised between 60 and 100 µm, preferably between 70 and 90 µm, and still more preferably of 80 µm, and/or the sodium aluminosilicate has an average grain size comprised between 2 and 15 µm, preferably between 5 and 10 µm, and still more preferably of 7 µm.

6. A foundry binder system, comprising the powder composition according to any of claims 1 to 5 and an aqueous solution of sodium silicate.

7. The binder system according to claim 6, wherein the aqueous solution of sodium silicate comprises from 25 to 50% by weight of sodium silicate, preferably from 35 to 45% by weight of sodium silicate, and from 50 to 75% by weight of water, wherein said percentages refer to the total weight of the aqueous solution of sodium silicate.

8. The binder system according to any one of claims 6 and 7, wherein the aqueous solution of sodium silicate comprises further additives.

9. The binder system according to claim 8, wherein the aqueous solution of sodium silicate comprises from 0.1 to 10% by weight of additives, wherein said percentages refer to the total weight of the aqueous solution of sodium silicate.

10. The binder system, according to any one of claims 6 to 9, wherein the aqueous solution of sodium silicate has a SiO₂/Na₂O molar ratio comprised between 1.9 and 2.5, preferably between 2.0 and 2.4, and more preferably between 2.1 and 2.3.

11. The binder system, according to any one of claims 6 to 10, wherein the aqueous solution of sodium silicate has a density at 20° C comprised between 1.30 and 1.50 kg/dm³, preferably between 1.35 and 1.45 kg/dm³.

12. Use of the binder system according to any one of claims 6 to 11, for the manufacture of foundry cores and/or moulds.

13. A moulding material mixture for the manufacture of foundry cores and/or moulds containing the binder systems according to any one of claims 6 to 11 and silica sand and/or other granular refractory material.

14. The moulding material mixture according to claim 13, comprising:

- from 1.6 to 2.4 parts, preferably from 1.8 to 2.2 parts, by weight, of the aqueous solution of sodium silicate as defined according to any one of claims 6 to 11;

- from 0.5 to 1.5 parts, preferably 0.7 to 1.2 parts, by weight, of the powder composition as defined according to any one of claims 1 to 5; and

- 100 parts by weight of silica sand and/or other granular refractory material.

15. A procedure for the production of foundry cores and/or moulds, comprising a production step of the moulding material mixtures according to any one of claims 13 to 14, and a heating step of said moulding material mixtures.

16. Foundry cores and/or moulds, obtainable according to the procedure of claim 15.