EP0044763A2 - Compositions for silicate-bonded foundry moulds and cores - Google Patents

Abstract

The present invention relates to new compositions for molds and cores in foundries, linked by a binder based on silicates. It is characterized by the fact that the cleaning agent comprises a metallic mineral derivative, in particular from the group of alkaline earths, in the form of a fine powder and of alumina. It is applied in foundry.

Description

The present invention relates to new compositions for molds and cores in foundries, linked by a binder based on silicates.

It has long been known to prepare cores and molds in foundries by using a ternary mixture comprising from 93 to 98 parts by weight of a sand suitable for the foundry industry, and from 2 to 7 parts by weight of '' an aqueous solution of alkaline silicate.

Such compositions were generally cured with carbon dioxide. This process has been improved by using different additives in patent FR 1 172 636. Finally, efforts have been made to avoid, especially for large parts, the use of carbon dioxide which has certain drawbacks, especially at high temperature.

Thus, in US 3,207,057, it has been proposed to use a binder comprising an aqueous solution of an alkali metal silicate and an additive consisting essentially of 3 to 100% of alumina relative to the binder.

However, it is known that many qualities of these compositions are required. In particular, we expect cores and molds with good mechanical qualities. We also want to obtain simultaneously a shorter setting time and a relatively long life time.

By life time is meant the time interval during which it is possible to store the sand + silicate + hardener mixture, without a reduction in mechanical properties being observed, from the moment when we have added silicate with sand + hardener mixture.

By setting time is meant the time interval which separates the moment when the silicate is added to the sand + hardener mixture from the moment when no changes are observed on the surface of the agglomerated sand. In practice, the moment of setting is appreciated simply by exerting pressure on said surface.

This is why it was proposed, in the French application 77 13177, on behalf of the plaintiff, to call upon cat Curing lysers of aqueous solutions of alkaline silicates based on alkylene carbonates, characterized in that they also contain methyl esters of organic acids.

But if such hardeners, used in combination with sand and binders based on alkaline silicate, lead to excellent results both with regard to the mechanical properties as well as the life and setting times, a problem still remains to be solved. not entirely satisfactory: it is the cleaning. By "cleaning" is meant the action which consists in fairly easily extracting from the metal part, the sand located inside the cavities formed by the cores.

Indeed, the agglomerated sand of the molds and cores must have a high mechanical resistance before the casting of the metal, and maintain a satisfactory mechanical resistance when hot; but once the metal has cooled, mechanical strength is fairly low so that the sand can be easily removed.

In order to facilitate cleaning, it has already been recommended to use carbonaceous materials and / or film-forming resinous adhesives in FR 2 237 706, but a phenomenon of recarburization may occur.

Also claimed in French application 77 38 182 compositions for molds and cores in foundry comprising sand and a binder based on an alkaline silicate, and a hardening agent which contain a cleaning agent consisting, at least in part, by an alumina of average particle size substantially between 0.2 and 5 μm.

These compositions give excellent results in the case in particular of treatments at high temperature, such as for example for cast iron or steel. Unfortunately, if the nature of the metal requires lower temperatures, for example 750 ° C, a decrease in the efficiency of the process is observed.

The present invention relates to compositions which make it possible to overcome this drawback. The compositions according to the invention comprise sand and an alkali silicate binder, as well as a hardening agent and a cleaning agent. They are characterized by the fact that the cleaning agent comprises a metallic mineral derivative, in particular from the group of alkaline earth, in the form of a fine powder and a hydrated alumina, of average particle size between 0.2 and 5 μm, and in particular between 0.5 and 3 μm.

It has in fact been observed that with certain mineral derivatives according to the invention there is synergy with such aluminas.

This is unexpectedly reflected in the fact that it is possible to improve both the mechanical properties and the friability of the compositions by substituting part of the alumina in an alumina composition with a cleaning agent according to the invention. . This is the case in particular when using calcium or magnesium oxides or carbonates with a particle size substantially less than 5 μm.

Also according to a preferred embodiment of the invention, the cleaning agent comprises from 1 to 99% hydrated alumina with an average particle size of between 0.2 and 5 μm and preferably between 0.5 and 3 μm, and from 1 to 99% of a metal derivative according to the invention.

In addition, the aluminas according to the invention advantageously have a B.E.T. less than 300 m2 / g preferably between 3 and 40 m2 / g.

According to a particular implementation of the invention, aluminas A1 ₂ 0 ₃ , 3H ₂ 0, hydrargillite type, can be used.

Conveniently, and preferably, the compositions according to the invention contain from 90 to 98 parts by weight of sand, 2 to 10 parts by weight of an alkali silicate solution, and from 0.5 to 5% by weight of alumina according to the invention, preferably from 0.8 to 1.7%, and from 0.5 to 5 parts of the additive according to the invention.

The silicate according to the invention preferably has a SiO ₂ / Na ₂ O weight ratio of between 2 and 2.7. A compound from the group of alkylene carbonates and / or organic esters, such as mono, diacetin, triacetin or mixtures thereof, glycol esters, in particular methyl esters, and in particular methyl esters of monoacids, may especially be used as hardener. organic, optionally substituted by other functional radicals, such as methyl lactate; they can be dimethyl esters of organic diacids, such as aliphatic diacids having three to ten carbon atoms, such as malonic acid, succinic acid, glutaric acid and adipic acid.

The cyclic alkylene carbonates have an alkylene radical preferably having two to ten carbon atoms, the carbonates usually used are ethylene carbonate and propylene carbonate.

In general, 4 to 30 parts by weight of organic acid methyl ester are used per 96 to 70 parts by weight of alkylene carbonate; occasionally, this mixture can be diluted with a solvent controlling the reactivity with the alkali silicate. As such solvents, aliphatic polyols and preferably aliphatic polyalkylene glycols such as diethylene glycol can be used. These solvents may be used for example in an amount of 2 to 20 parts by weight to 100 _'parts by weight of alkylene carbonate hardener / methyl ester.

However, other types of hardeners can also be used, such as carbon dioxide or blast furnace slag.

The processes for obtaining a foundry mold or core are in accordance with the usual technique, as for example described in US 3,307,046 or FR 2,264,608. In particular, it is possible to operate at room temperature , i.e. roughly between 0 and 30 ° C.

In the following examples, the procedure is as follows:

• - du sable SIFRACO (référence AFA 53-57)
• - le débourrant
• - le durcisseur
• - la solution aqueuse de silicate de sodium. Cette solution de silicate de sodium a une teneur en eau de55 % et un extrait sec de 45 %. Le rapport pondéral SiO₂/Na₂O est égal à 2,39, sa densité à 20°C est de 1,525 et sa viscosité de 600 millipascals par seconde à 20°C.

In a KENWOOD planetary mixer, the components of the molding mixture are introduced at a temperature between 18 and 20 ° C in the following order:

• - SIFRACO sand (reference AFA 53-57)
• - the breaking
• - the hardener
• - the aqueous sodium silicate solution. This sodium silicate solution has a water content of 55% and a dry extract of 45%. The SiO ₂ / Na ₂ O weight ratio is 2.39, its density at 20 ° C is 1.525 and its viscosity at 600 millipascals per second at 20 ° C.

The sand used has the following characteristics: specific surface 115 cm2 / g; bulk density 1.5; loss on ignition 0.15 X. It contains a minimum of 99.7% of SiO ₂ and a maximum of 0.1% clay with traces of calcium carbonate.

• - 1 % supérieur à 420 µm
• - 26 % compris entre 420 et 300 µm
• - 47 % compris entre 300 et 210 µm
• - 23 % compris entre 210 et 150 µm
• - 3 % compris entre 150 et 105 µm

Its particle size distribution is:

• - 1% greater than 420 µm
• - 26% between 420 and 300 µm
• - 47% between 300 and 210 µm
• - 23% between 210 and 150 µm
• - 3% between 150 and 105 µm

Compression resistance test according to technical sheet from the Technical Center for Foundry Industries, Sand Molding Studies Division. July 1971.

After preparation. from the molding mixture (sand + settling + hardener + silicate) to the KENWOOD planetary mixer, it is made up in 4 minutes in a box of 7 test tubes. These comply with DIN 52-401 (type G.F.) and measure their compressive strength at time intervals between 10 minutes and 1 hour, using the GF resistance device, type SPDR. The origin of the times is fixed at the time of the silicate introduction.

Shear resistance test according to technical sheet of the Technical Center for Foundry Industries, Sand Molding Studies Division. July 1971 by producing test pieces in accordance with DIN 52-401 (type G.F.)

Friability test

The test piece is brought to the indicated temperature of 500 ° C, 750 ° C and 1000 ° C in an oven for 30 minutes, then after cooling. We place it in a tube where we drop a mass of 600 g from a height of 50 cm. Under the effect of the shock the test tube bursts. The pieces are sieved through a 2 mm mesh sieve for 10 seconds. The rejection weight / initial weight ratio is considered to represent the friability F. If F is close to 0, we consider that it is a good breaking-in and if F is close to 1 the breaking properties are zero.

Recall that a good breaker must not alienate the mechanical properties, indicated above, of the test piece.

• 80 % du mélange ester adipate de méthyle ester glutarate de méthyle ester succinate de méthyle
• 20 % de diéthylène glycol.

In Example 1, the hardener has the following composition:

• 80% of the mixture methyl adipate ester methyl glutarate ester methyl succinate ester
• 20% diethylene glycol.

Alumina is a hydrargillite, Al ₂ O ₃ , 3H ₂ 0 with an average particle size equal to 1 μm, with 5 m2 / g of specific surface area, containing 35% of water of crystallization.

Example 1

• - The mixing is carried out as follows
• - 1 kg of sand
• - 1.8% breaking-in
  1st mixing of 2 minutes.
• - 0.5% hardener
  2nd mixing of 2 minutes.
• - 3.5% sodium silicate
  3rd mixing of 2 minutes.

Dans cet exemple on a mesuré la résistance à la compression après 24 heures parce que la prise n'est pas immédiate et la mesure ne devient significative qu'après 24 heures.The results obtained are summarized in Table 1 below.

In this example, the compressive strength was measured after 24 hours because the setting was not immediate and the measurement only became significant after 24 hours.

Example 2

This example is identical to the previous one except that the hardener according to example 1 is replaced by a gassing of C0 ₂ for 10 seconds under 2 10 ⁵ _Pa .

The shear strength is given here because the setting is immediate and the shear strength is more significant than the compressive strength.

It can therefore be seen in these examples that the partial substitution of alumina by calcium carbonate or magnesium oxide leads both to improvement of the friability and at least to maintenance of the mechanical properties. It appears that when using alumina alone, the settling properties around 750 ° C. are not entirely satisfactory. In addition, when using magnesium carbonate alone, the mechanical properties appear insufficient.

The compositions according to the invention which comprise both alumina and a mineral derivative from the group of alkaline earths, remedy the aforementioned drawbacks.

Example 3

• malonate de diméthyle 40
• malonate de diéthyle 60
• avec un mélange 50 % alumine 50 % carbonate Ca.
• On observe à 750°C une valeur de la friabilité de 0,07.

This example is identical to Example 1 except that the hardener of Example 1 is replaced by the following hardener:

• dimethyl malonate 40
• diethyl malonate 60
• with a mixture of 50% alumina 50% carbonate Ca.
• A friability value of 0.07 is observed at 750 ° C.

Example 4

In this example according to Example 3, the hardener consists of a mixture of 85% diacetin and 15% triacetin.

Friability at 750 ° C = 0.05

Example 5

In this comparative example, on the one hand, the magnesium carbonate alone is used according to example 2 and on the other hand, a mixture of 50% alumina, 50% magnesium carbonate according to example 2. The compressive and shear strengths on test pieces produced at increasing times since the preparation of the mixture so as to determine the lifetime of the mixture.

The results obtained are collated in the table below:

It appears that, unlike the mixtures according to the invention, magnesium carbonate has a too short life time to allow easy industrial processing.

Claims (9)

1) New compositions for foundry molds and cores comprising sand and a binder based on an alkaline silicate, a hardening agent and a cleaning agent, characterized in that the cleaning agent comprises a metallic mineral derivative, in particular from the group of alkaline earths, in the form of a fine powder and between 1 and .99% of a hydrated alumina with an average particle size between 0.2 and 5 μm and in particular between 0.5 and 3 μm. 2) New compositions according to claim 1, characterized in that the mineral derivative is from the group of oxides and carbonates of Ca and Mg. 3) New compositions according to claim 1, characterized in that they contain aluminas Al ₂ O ₃ , 3 H ₂ 0. 4) New compositions according to one of the preceding claims, characterized in that they contain: - 90 to 98 parts by weight of sand - from 2 to 10 parts by weight of alkali silicate - from 0.5 to 5% by weight, preferably from 0.8 to 1.7% of alumina. - from 0.5 to 5% breaking-in. 5) New compositions according to one of the preceding claims, characterized in that the binding agent consists of an alkali silicate solution with a weight ratio SiO ₂ Na ₂ O of between 2 and 2.7. 6) New compositions according to one of the preceding claims, characterized in that the hardening agent consists of at least one compound from the group of alkylene carbonate and organic esters, in particular methyl esters. 7) New compositions according to claim 6, characterized in that the hardening agent comprises from 4 to 30 parts by weight of organic acid methyl ester per 96 to 70 parts by weight of alkylene carbonate. 8) New compositions according to one of the preceding claims, characterized in that the hardening agent consists of carbon dioxide. 9) New compositions according to one of the preceding claims, characterized in that the hardening agent consists of a blast furnace slag.