GB2237015A - Method of manufacturing glass-ceramic articles - Google Patents

Abstract

Glass ceramic articles of a defined shape, particularly thin products and products with sharp corners, are made by forming the shape of the desired article from a mixture of powdered glass particles and binder, and then heating the shaped mixture at the liquidus temperature of the glass of the glass particles for a time sufficient for the glass particles to crystallise across the boundaries of contacting glass particles.

Description

METHOD OF MANUFACTURING GLASS-CERAMIC ARTICLES This invention relates to a method of manufacturing glass-ceramic articles and to the articles made by the method.

It is well known that glass-ceramic products may be made by producing a molten glass from glass-forming constituents, or pieces of glass, which include a nucleating agent, forming the molten glass into a desired shape, for example by pressing into slabs, and then heating the formed glass shape at a temperature at which crystallisation occurs.

However the products obtained in this way always tend to have round edges as a result of the surface tension effect in the molten glass. Also it is difficult to form thin products and very difficult if not impossible to form a thin product having sharp corners.

Hitherto it has been thought that, when making a glass-ceramic article from discrete pieces of glass, it is necessary first to heat the glass particles to the melting temperature of the glass.

I have now made the surprising discovery that a glass-ceramic article may be produced by adhering together particles of powdered glass using a binder, and then heating the glass particle/binder mixture at the liquidus temperature of the glass.

In accordance with the present invention therefore there is provided a method of manufacturing glass-ceramic articles comprising the steps of mixing powdered glass particles with a binder, forming the mixture of powdered glass particles and binder into the shape of a desired article, and heating the shaped mixture at the liquidus temperature of the glass of the glass particles for a time sufficient for the glass particles to crystallise across the boundaries of contacting glass particles and thereby produce a glass-ceramic article of the desired shape.

By use of the method of the present invention, exact shapes can be made uninhibited by the tendency of molten glass to form round edges. In addition the method enables a thinner product to be made than was possible by prior art processes and, in particular, the method in accordance with the present invention enables the production of thin products with sharp corners.

Preferably the powdered glass particles are of a size to pass an 850 micron sieve although smaller particle sizes may be used. The finer the powder used, the more dense is the final product. However smaller particle sizes give rise to shrinkage so that the size of the final article is less than the size of the shaped mixture taken from the mould in which the mixture was shaped. It has been found that, when the method in accordance with the present invention is performed using glass particles of a size to pass a 250 micron sieve, the article shrinks in size by about 10% although the article retains its shape and sharp corners. For smaller particle sizes the degree of shrinkage increases and for particle sizes of the order of 10 microns the shaped mixture will shrink by about 30% in size, although the desired shape is retained.The final articles are, in all cases, glass-ceramic articles of considerable strength and of high resistance to abrasion.

The binder used may be an organic binder which is destroyed during the heat treatment.

The process in accordance with the present invention has particular application in forming glass-ceramic articles from a glass frit and more specifically from glass frits containing heavy metals prepared as described in my co-pending Patent Applications Nos. 8903224.7 and In the said co-pending patent applications I have described a method of treating toxic waste resulting from the incineration of industrial or domestic refuse in order to render the material harmless by forming it into a glass without releasing volatilised heavy metals and heavy metal compounds into the atmosphere.However in order to prevent volatilisation of the heavy metals from the molten glass when the molten glass is removed from the furnace, the molten glass must be quenched as quickly as possible by pouring it into water within an enclosed space from which volatilised metals and heavy metal compounds cannot escape. A method and apparatus for achieving this is particularly described in my copending Patent Application No. , and the resulting product is a glass frit.

When subjected to the internationally accepted standard test for leaching of heavy metals, German D.E.V.S. 4, the glass frit was shown to be acceptable for use as a product.

Although this glass frit may be used as such in products, for example in concrete, as an aggregate, in road foundation, or as an asphalt filler, it is desirable to have further outlets for this safe waste product which will be produced in very large quantities because of the large quantities of refuse which have to be incinerated in modern industrial countries.

The toxic waste resulting from incineration of industrial or domestic refuse includes nucleating agents which are consequently also present in the glass frit product of the said co-pending patent applications. This glass frit product may be re-melted in a conventional cold-top furnace and fed via a forehearth to production machines for making the glass into tiles and pipes which are then re-heated to crystallise the glass. These crystallised glasses or slags are then more resistant to abrasion, chemical attack and leaching. They are also strong materials which are not subject to fracture in the way that normal glass products may fracture.

In accordance with the invention of the present application, the said glass frits are, as a further alternative, used as a raw material in forming glassceramic articles by a method in which powdered glass particles are caused to crystallise together at their points of contact by heating at the liquidus temperature of the glass.

The present invention will be further understood from the following detailed description.

Glass frit obtained from toxic waste by the method of my co-pending Patent Applications Nos. 8903224.7 and is crushed to a powder. This powder is treated by a magnet to remove all free iron. Because the toxic wastes used in the method of the said co-pending patent applications usually contain carbon and other reducing agents as well as iron oxides, free iron metal and alloys are produced during melting. While some of this free metal lies on the bottom of the melter and is extracted therefrom, other free metal will be carried into the frit and it is this free metal which must be removed by a magnet.

The powder is then screened to a desired particle size and the screened powder is mixed with a binder to enable it to hold a shape prior to and during firing.

The binder may be either an organic binder such as ethanol, starch, lignates or isopropanol, or an inorganic binder such as aluminium chloride, a phosphate or ethylsilicate. After mixing with binder the powder/binder mixture is shaped by any conventional method, such as machine pressing, rolling, vacuum forming or extrusion, and the shaped powder/binder mixture is then heat treated to form a solid product by heating to a critical temperature for the glass composition, usually a maximum temperature of 1100 C, and holding the shaped mixture at this temperature for a period of the order of two hours so that a crystalline structure is fully developed by crystallisation across the contacting boundaries of the particles of the glass powder.

Because there are a variety of sources for the toxic waste treated by the method of my co-pending Patent Applications Nos. 8903224.7 and , the glass frits derived therefrom have varying compositions.

However, the glass frits derived from different sources are conveniently put into large heaps from which blended material of approximate known chemical composition may be extracted.

Glass frits were prepared by the method of my said co-pending patent applications from three batch compositions as follows: 1 2 3 Toxic Waste Fly Ash 40 pts 21 pts 40 pts Limestone 50 pts - 15 pts Magnesite (Mg 0) 4 pts 4 pts 40 pts Power Station Fly Ash (P.F.A.) 100 pts 20 pts 100 pts Sand (SiO2) - 20 pts Residual Lime after Gas Washing 35 pts Advantageously most ingredients of the batch compositions are themselves waste products, for example the magnesite may be obtained from the steel industry and the sand may be waste from grinding processes.

The three glass frits were then each divided into three separate samples and one of each of the three samples was treated by each of the three following processes: Process A The particles were crushed and sieved to extract all particles not passing an 850 micron sieve, the screened particles were mixed with a binder which was aluminium chloride or isopropanol, the particle/binder mixture was machine pressed under a pressure of 100 tons to produce a desired shape and the shaped mixture was fired at 11200C for two hours. All three compositions were found to maintain their original shape and size, no shrinkage of the shaped mixture being evident after firing.

Process B This process was the same as Process A except that the powdered frit was screened using a 250 micron sieve and only particles passing the 250 micron sieve were mixed with the aluminium chloride or isopropanol binder. After firing, all three compositions retained their shape and sharp corners but shrank by about 10%.

Process C This was the same as Process A except that the powdered frits were screened using a 10 micron sieve and the particles passing the 10 micron sieve were mixed with isopropanol as binder. Again all three compositions were found to retain their original shape but they all shrank by about 30%.

The products of all these processes were very dense glass-ceramic products with high resistance to abrasion and high strength.

In all the processes described, glass crystals have grown across the boundaries of contacting glass particles at a temperature below the melting temperature of the glass which in each case is of the order of 13500C.

The growth of crystals in this way at a temperature below the glass melting temperature is a very surprising achievement.

Products made by the process of the present invention easily pass the D.E.V.S. 4 test for acceptability against leaching.

Claims (8)

CLAIMS:

1. A method of manufacturing glass-ceramic articles comprising the steps of mixing powdered glass particles with a binder, forming the mixture of powdered glass particles and binder into the shape of the desired article, and heating the shaped mixture at the liquidus temperature of the glass of the glass particles for a time sufficient for the glass particles to crystallise across the boundaries of contacting glass particles and thereby produce a glass-ceramic article of the desired shape.

2. A method according to Claim 1 wherein the powdered glass particles are derived from a glass frit containing heavy metals.

3. A method according to Claim 1 or Claim 2 wherein the powdered glass particles are mixed with an inorganic binder.

4. A method according to Claim 1 or Claim 2 wherein the powdered glass particles are mixed with an organic binder.

5. A method according to any one of the preceding claims wherein the shaped mixture of powdered glass particles and binder is heated at a temperature of the order of 11000C for a period of two hours.

6. A method according to Claim 5 wherein the shaped mixture is heated at a temperature of 11200C.

7. A method of manufacturing glass-ceramic articles substantially as hereinbefore described.

8. Glass-ceramic articles prepared by a method in accordance with any one of the preceding Claims.