Binder compositions for bonding particulate material

Details Binder compositions for bonding particulate material Binder compositions for bonding particulate material

2000-08-24

2002-07-09

Marcantoni, Paul (Department: 1755)

Compositions: coating or plastic

Coating or plastic compositions

Molds and mold coating compositions

C106S038200, C106S038220, C106SDIG001, C252S062000

Reexamination Certificate

active

06416572

ABSTRACT:

The present invention relates to binder compositions for bonding particulate materials. The invention has particular utility in the foundry industry, for forming bonded particulate articles, including foundry moulds and cores, and other refractory articles for use with hot molten metal, e.g. linings and feeder sleeves, including insulating, exothermic, and duplex (i.e. insulating and exothermic) sleeves.
The formation of foundry moulds and cores from bonded particulate refractory material, e.g. sand, is very well known. It is also very well known to form other refractory articles such as ladle linings, feeder head linings, feeder sleeves and the like, from bonded particulate materials. A feeder sleeve provides a reservoir for molten metal and enables the molten metal to remain molten longer than a casting in which it is being employed. The feeder sleeve thus enables the molten metal to continue to feed the casting as it solidifies, providing for a sound and strong casting. Refractory articles, such as linings and feeder sleeves consequently are often formed from insulating materials, to reduce heat losses. Some applications (such as feeder sleeves) involve the use of consumable insulators whilst others require insulators that are durable and able to repeatedly cycle through a range of temperatures. High grade, low density insulators (typically 0.5 g/cc) are known and are based on ceramic fibre. High density products, based on silica, typically have an open porous structure.
Feeder sleeves are produced by a variety of methods, including the resin bonding of waste silaceous materials such as so-called “flyash floaters” (sometimes known by the trade marks “Extendospheres” or “Cenospheres”). Foundry moulds and cores are often produced by the resin bonding of silica and/or other sand. Resin bonding is generally employed because, when the sleeves, moulds or cores are gas cured in a pattern box, the resin enables good strength and dimensional accuracy to be achieved. However, in the presence of molten metal the resins employed normally generate considerable amounts of fumes and gases. In some circumstances, this fume and gas is absorbed by the molten metal, leading to a deterioration in its quality. The fume problem is particularly problematic in the casting of low temperature alloys, for example those including aluminium where there is insufficient molten metal heat to burn the resins, but sufficient molten metal heat to volatise the components as smoke and fume. It would be advantageous if bonded particulate refractory articles, including moulds, cores and feeder sleeves, could be produced to good dimensional accuracy but without the problem of fume and smoke generation.
In a first aspect the present invention provides a method of producing a bonded particulate material comprising the steps of:
combining an alkali with a particulate metal oxide that is capable of forming a metalate in the presence of the alkali; and
drying the particles after a portion of each metal oxide particle has formed the metalate, in a manner such that an unreacted particle core remains after drying.
By maintaining a metal oxide core of each metal oxide particle in the resultant bonded particulate material, a refractory and/or insulating function can generally be provided and yet high dimensional stability and accuracy can normally be achieved. Also, a high degree of bonding between adjacent metal oxide particles can generally be achieved because the exterior surface of the metal oxide particles typically “dissolves”, thus enabling a bond to form between adjacent metal oxide particles, and which bond “solidifies” after drying.
When the term “metal” is used in the present specification it is intended to include quasi metals such as silicon. When the expression “metal oxide” is used, it is used in relation to a solid metal oxide that is typically capable of use as a refractory material, an insulating material, a construction material, or other bonded particulate material. When the expression “particulate material” is used herein, it includes within its scope fibrous material and/or granular material and/or powder material and/or fines etc. The term “metalate” is used herein to refer to oxo anions (also known as “oxyanions”) which may be considered as being formed by the co-ordination of oxide O
2−
ions with metal (including quasi metal, such as silicon) cations to form metal-and-oxygen anions, possibly including hydroxide groups, especially under alkaline conditions. These are the normal species in aqueous solution, however. their exact structures are often complex rather than simple discrete species, with typical examples including silicates, titanates, aluminates, zincates, germanates, etc. Such metal-and-oxygen anions (metalates) are then associated with alkali metal cations (such as Na
+
or K
+
) from the alkali.
Most typically the alkali is in the form of an aqueous solution, such that “drying” involves driving off water from the mixture of the metal oxide and the alkali solution. However, if the reaction were conducted in the gas or molten phase, “drying” would mean adjusting the conditions to cause the reaction between the metal oxide and the alkali to stop.
Metal oxides that are preferably employed (and which typically function as a binding material) include silica fume, fine alumina, fine titania, zinc oxides etc (the use of “fine” referring to a fine particulate form of the oxide). These materials readily form a metalate in the presence of an alkali solution.
Preferably these types of metal oxides function as a “binder” in the insulator produced in accordance with the invention.
The metal oxide can also be a waste siliceous material, such as flyash, flyash floaters (FAF) or other oxidisable waste oxide; thus, a valuable product can be produced using waste material. (Flyash floaters are hollow microspheres of silica and/or alumina—they normally comprise aluminosilicate, possibly with other constituents.) A variety of other metal oxides can be employed. For example, silica sand, bauxite, alumina, perlite, etc can be employed. However, usually these latter materials (ie. including FAF) constitute a “filler” component of the bonded particulate material, and form the “bulk” of the bonded particulate material rather than providing the major binding function. The filler, or a combination of fillers, is then typically used in conjunction with and bonded by a binding material (as defined above). It should be appreciated, however, that either metal oxide binders or fillers can be used on their own to form the bonded particulate material. Also, some filler employed in the present bonded particulate materials may not have a reactive oxide consistency and hence may only form a relatively weaker bond with a binding material. Preferably when non-oxide fillers are employed they can still bind with a metalate, such as a silicate, aluminate, titanate, zincate etc.
Preferably the alkali is a solution produced from a strong alkali such as sodium hydroxide, potassium hydroxide, lithium hydroxide etc. Sodium hydroxide is most preferred because of its relative abundance and low cost.
In a preferred variation of the method, the binder material is premixed with the alkali solution prior to mixing the filler material therewith. When the alkali solution is based on sodium hydroxide, employing a premix can minimise the amount of alkali solution required. Because sodium acts as a flux in bonded particulate materials, it is desirable to minimise its presence in the resulting bonded particulate material, and it has been observed that the formation of the premix assists in reducing the quantity of sodium present in the resulting bonded particulate material.
Preferably the drying step is conducted in a microwave oven or urn. Microwave radiation has been observed to be an expedient way of achieving drying and forming a bond, thus maintaining a metal oxide core. However, conventional convection and radiation ovens and urns can be employed as can dielectric heating. Additionally or alternatively, a heated core box m

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