Process for preparing particles covered with a layer of...

Details Process for preparing particles covered with a layer of... Process for preparing particles covered with a layer of...

1999-06-18

2001-06-19

Tentoni, Leo B. (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Direct application of electrical or wave energy to work

Forming articles by uniting randomly associated particles

C106S038300, C106S038350, C164S045000, C264S071000, C264S219000

Reexamination Certificate

active

06248284

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a process for preparing particles covered with a layer of water glass and a process for preparing articles comprising such covered particles. Furthermore, the invention concerns such particles and articles obtainable by these processes.
When casting metal and plastic objects, moulds and cores of various materials are used. The present invention is directed to such cases where the main ingredient of the material in the moulds and cores is constituted by solid particles such as sand.
BACKGROUND
To adhere the solid particles, different types of binders have been used. Such binders can be of either organic or inorganic nature. Organic binders are usually phenol-based and to a large extent preferred by iron and metal foundries. However, phenol-based binders are detrimental to the environment as during the casting, the heating of phenol-based binders entails pyrolysis whereby CO and polyaromatic hydrocarbons (PAH) are released, among others. Furthermore, the casting leaves some sand which is polluted with toxic organic compounds. The European iron and metal foundries alone produce 7,000,000 tons of polluted sand every year.
Consequently, extensive research has been performed to provide inorganic binders. The use of water glass as a binder has been suggested but so far, the use of water glass as a binder has not been widespread within the iron and metal casting industry.
Water glass as a binder was used to adhere sand in foundries for the first time about 1920. In this process, sand is mixed with an aqueous solution of water glass, and this mixture is subsequently filled into a mould to be cured by evaporation of the water. This process was essentially improved about 1950 by the development of the CO
2
process in which the CO
2
gas is led through the wet mixture of sand and water glass in order to cure the binder. Cores and moulds obtained by the CO
2
process are relatively porous and their strength is weak.
At the end of the 1960s, self-curing binding systems were developed. The curing is effected by supplying an organic ester to the wet mixture of sand and water glass before filling it into a mould.
At the end of the 1970s, there was developed a dry curing process where energy from warm air, a warm mould, or microwaves entail curing of the water glass binder.
In the early nineties, the present inventor developed a curing process where warm air was blown into a ventilated mould by use of a three-step temperature cycle, cf. “Miljøprojekt nr. 189, Miljøstyrelsen”.
All the aforesaid processes in which water glass is used as a binder have, however, the inconvenience that the flowing properties of the mixture of sand, water glass, and water are relatively poor when filling the mould. This entails that the mixture tends not to flow into all the cavities of the mould. The known processes using water glass are thus limited to be used in connection with moulds showing no detailed surface.
SUMMARY
Therefore, it is the object of the present invention to procure a process for preparing a free flowing product which can be used to fill moulds with a detailed surface. It is further the object to procure a process for preparing an article comprising such a free flowing particle which article can be obtained with a detailed outer design.
Consequently, the present invention relates to a process for preparing particles covered with a layer of water glass, which process is characterized by the steps of
(a) providing a mixture containing particles to be covered, water and 0.1-5% by weight water glass, calculated on the basis of the weight of the particles, the water glass being dissolved in said mixture, and
(b) stirring the mixture mechanically, optionally by supplying heat from an extern heat source, and permitting the water to evaporate from the mixture until at least so much water is evaporated that said mixture is no longer sticky.
The invention further concerns particles covered by an essentially unbroken layer of water glass obtainable by this process.
DETAILED DESCRIPTION
The term “water glass” in the present description and claims is to be understood as sodium or potassium silicates. The sodium or potassium component (M
2
O, M Na or K) can be in variable quantities in relation to the silicate component (SiO
2
). The ratio SiO
2
/M
2
O is designated the weight module. A water glass with a low weight module is easily soluble in water and strongly basic because a high content of the basic component M
2
O. Correspondingly, a water glass possessing a high weight module, has a smaller basic character, and a smaller water solubility. In this invention, the use of water glass with a weight module between 0.5 and 4, especially betwen 1.8 and 3.5, is preferred. Furthermore, M═Na is preferred.
For water glass with a weight module of up till about 3.0, it is possible to dissolve this immediately by contact with water at room temperature within a reasonable period of time, e.g. within 10 min. For water glass with a weight module over about 3.0, e.g. module 3.0 to 3.5, it is appropriate to predissolve the solid water glass in water with a temperature of up till 100° C. or to heat the particles to a temperature of up till 100° C., preferably 80-90° C., before supplying water and water glass to ensure an essentially completely dissolving of the water glass.
The particles to be covered with a layer of water glass can be any material to which water glass can adhere.
Examples of such materials are metallic materials and ceramics. Examples of usable metallic materials are aluminium, copper, iron, tungsten, chrome, vanadium and manganese. Examples of ceramics are SiO
2
(quartz sand), TiO
2
, ZrO
2
, Al
2
O
3
, CaO and MgO.
The size and the size distribution of the particles are not essentiel to the production of the covered particles according to the invention. For use in mould and core boxes, it is in general preferred to use particles with a size of from 0.05 mm to 2.0 mm, especially 0.10 to 0.60 mm. The selected particles can have a broad or narrow particle size distribution or the particle mass can be composed by socalled double sieved particles (double sieved sand) in which there are two peaks on the grain size curve.
The quantity of water in the mixture in step (a) depends on several factors, such as the specific surface, porosity and the electrostatic nature of the particles, but is preferably at least 0.1% by weight, calculated on the weight of particles to be covered. A quantity of typical 1-3% by weight is chosen to ensure a sufficient humidification of the particles. The quantity of water does preferably not surpass 5% by weight as supplying water in surplus of this quantity will not contribute further to the humidification of the surface of the particles to be covered.
The quantity of water glass in the mixture comprising particles to be covered, water, and water glass, depends on factors such as the desired thickness of the layer and the specific surface of the particles to be covered. According to the invention, 0.1-5% by weight water glass can be used, calculated on the basis of the weight of the particles. It is in general preferred to use 1-3% by weight.
The mixture in step (a) is obtained preferably by the steps of
(a1) mixing water and particles to be covered,
(a2) stirring the mixture such that the water is distributed evenly in the particle mass,
(a3) supplying 0.1-5% by weight water glass to this mixture, and
(a4) continuing the stirring until the water glass is evenly distributed and dissolved.
In step (a2) the water glass is preferably supplied to the mixture in solid state, it being preferred to use particles of water glass prepared by spray drying.
It appeared expedient to effect a stirring during the entire course of production of the particles covered with a layer of water glass. This stirring is mechanical, preferably by use of rotating wings. The stirring velocity is adjusted to ensure that no cured lumps of particles are formed which subsequently have to be broken.
As the adhesion tendency of the mixture increases strongly dur

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